moodle-report_usergrades/classes/usergrades_form.php

<?php

/**
 * Form class for user selection
 */
class usergrades_form extends moodleform
{
    public function definition()
    {
        $mform = $this->_form;

        // User selector
        $user_selector = new user_selector('userid', array('multiselect' => false));
        $users = $user_selector->find_users('');

        $mform->addElement('select', 'userid', get_string('selectuser', 'report_usergrades'), $users[get_string('users')]);

        // XLSX Export checkbox
        $mform->addElement('checkbox', 'exportxls', get_string('exportxls', 'report_usergrades'));

        // Add form action buttons
        $this->add_action_buttons(false, get_string('showgrades', 'report_usergrades'));
    }
}