diff --git a/auth.php b/auth.php
index 31df400..8a152f9 100644
--- a/auth.php
+++ b/auth.php
@@ -27,6 +27,7 @@ defined('MOODLE_INTERNAL') || die();
 use auth_userkey\core_userkey_manager;
 use auth_userkey\userkey_manager_interface;
 
+require_once($CFG->libdir . "/externallib.php");
 require_once($CFG->libdir.'/authlib.php');
 
 /**
@@ -182,4 +183,81 @@ class auth_plugin_userkey extends auth_plugin_base {
         return $CFG->wwwroot . '/auth/userkey/login.php?key=' . $userkey;
     }
 
+    /**
+     * Return a list of mapping fields.
+     *
+     * @return array
+     */
+    public function get_allowed_mapping_fields() {
+        return array(
+            'username' => 'username',
+            'email' => 'email',
+            'idnumber' => 'idnumber',
+        );
+    }
+
+    /**
+     * Return a mapping parameter for request_login_url_parameters().
+     *
+     * @return array
+     */
+    protected function get_mapping_parameter() {
+        $mappingfield = $this->get_mapping_field();
+
+        switch ($mappingfield) {
+            case 'username':
+                $parameter = array(
+                    'username' => new external_value(
+                        PARAM_USERNAME,
+                        'Username'
+                    ),
+                );
+                break;
+
+            case 'email':
+                $parameter = array(
+                    'email' => new external_value(
+                        PARAM_EMAIL,
+                        'A valid email address'
+                    ),
+                );
+                break;
+
+            case 'idnumber':
+                $parameter = array(
+                    'idnumber' => new external_value(
+                        PARAM_RAW,
+                        'An arbitrary ID code number perhaps from the institution'
+                    ),
+                );
+                break;
+
+            default:
+                $parameter = array();
+                break;
+        }
+
+        return $parameter;
+    }
+
+    /**
+     * Return user fields parameters for request_login_url_parameters().
+     *
+     * @return array
+     */
+    protected function get_user_fields_parameters() {
+        // TODO: add more fields here when we implement user creation.
+        return array();
+    }
+
+    /**
+     * Return parameters for request_login_url_parameters().
+     *
+     * @return array
+     */
+    public function get_request_login_url_user_parameters() {
+        $parameters = array_merge($this->get_mapping_parameter(), $this->get_user_fields_parameters());
+
+        return $parameters;
+    }
 }
diff --git a/externallib.php b/externallib.php
index a0863c3..a7f0c6a 100644
--- a/externallib.php
+++ b/externallib.php
@@ -28,49 +28,16 @@ require_once($CFG->dirroot . "/auth/userkey/auth.php");
 class auth_userkey_external extends external_api {
 
     public function request_login_url_parameters() {
-        // TODO based on settings set required parameter for one of the following: username, email, idnumber.
         return new external_function_parameters(
             array(
                 'user' => new external_single_structure(
-                    array(
-                        'username' => new external_value(
-                            PARAM_USERNAME,
-                            'Username policy is defined in Moodle security config.'
-                        ),
-                        'email' => new external_value(
-                            PARAM_EMAIL,
-                            'A valid and unique email address'
-                        ),
-                        'idnumber' => new external_value(
-                            PARAM_RAW,
-                            'An arbitrary ID code number perhaps from the institution'
-                        ),
-                        'firstname' => new external_value(
-                            PARAM_NOTAGS,
-                            'The first name(s) of the user',
-                            VALUE_OPTIONAL
-                        ),
-                        'lastname' => new external_value(
-                            PARAM_NOTAGS,
-                            'The family name of the user',
-                            VALUE_OPTIONAL
-                        ),
-                        'customfields' => new external_multiple_structure(
-                            new external_single_structure(
-                                array(
-                                    'type'  => new external_value(PARAM_ALPHANUMEXT, 'The name of the custom field'),
-                                    'value' => new external_value(PARAM_RAW, 'The value of the custom field')
-                                )
-                            ), 'User custom fields (also known as user profile fields)', VALUE_OPTIONAL
-                        ),
-                    )
+                    get_auth_plugin('auth_userkey')->get_request_login_url_user_parameters()
                 )
             )
         );
     }
 
     public function request_login_url($user) {
-
         $auth = get_auth_plugin('auth_userkey');
         $loginurl = $auth->get_login_url($user);
 
diff --git a/tests/auth_plugin_test.php b/tests/auth_plugin_test.php
index 62dd523..ac8fb9f 100644
--- a/tests/auth_plugin_test.php
+++ b/tests/auth_plugin_test.php
@@ -42,6 +42,8 @@ class auth_plugin_userkey_testcase extends advanced_testcase {
      */
     public function setUp() {
         global $CFG;
+
+        require_once($CFG->libdir . "/externallib.php");
         require_once($CFG->dirroot . '/auth/userkey/tests/fake_userkey_manager.php');
         require_once($CFG->dirroot . '/auth/userkey/auth.php');
 
@@ -219,4 +221,66 @@ class auth_plugin_userkey_testcase extends advanced_testcase {
         $this->assertEquals($expected, $actual);
     }
 
+    public function test_get_allowed_mapping_fields_list() {
+        $expected = array(
+            'username' => 'username',
+            'email' => 'email',
+            'idnumber' => 'idnumber',
+        );
+
+        $actual = $this->auth->get_allowed_mapping_fields();
+
+        $this->assertEquals($expected, $actual);
+    }
+
+    public function test_get_request_login_url_user_parameters() {
+        // Check email as it should be set by default.
+        $expected = array(
+            'email' => new external_value(
+                PARAM_EMAIL,
+                'A valid email address'
+            ),
+        );
+
+        $actual = $this->auth->get_request_login_url_user_parameters();
+        $this->assertEquals($expected, $actual);
+
+        // Check username.
+        set_config('mappingfield', 'username', 'auth_userkey');
+        $this->auth = new auth_plugin_userkey();
+
+        $expected = array(
+            'username' => new external_value(
+                PARAM_USERNAME,
+                'Username'
+            ),
+        );
+
+        $actual = $this->auth->get_request_login_url_user_parameters();
+        $this->assertEquals($expected, $actual);
+
+        // Check idnumber.
+        set_config('mappingfield', 'idnumber', 'auth_userkey');
+        $this->auth = new auth_plugin_userkey();
+
+        $expected = array(
+            'idnumber' => new external_value(
+                PARAM_RAW,
+                'An arbitrary ID code number perhaps from the institution'
+            ),
+        );
+
+        $actual = $this->auth->get_request_login_url_user_parameters();
+        $this->assertEquals($expected, $actual);
+
+        // Check some junk field name.
+        set_config('mappingfield', 'junkfield', 'auth_userkey');
+        $this->auth = new auth_plugin_userkey();
+
+        $expected = array();
+
+        $actual = $this->auth->get_request_login_url_user_parameters();
+        $this->assertEquals($expected, $actual);
+    }
+
 }