WPMU M2 Autofill username in registration form

code.php

<?php

add_action( 'wp_footer', function() {
        if( 'membership_signup' == $_POST['action'] )
        {
                $membership_id = $_POST['membership_id'];
                $membership = MS_Factory::load( 'MS_Model_Membership', intval( $membership_id ) );
                $prefix = str_replace( ' ', '_', strtolower( $membership->name ) ) . '_';
                $username = get_unique_username( $prefix );
                ?>
                <script type="text/javascript">
                jQuery( function($) {
                        $( '#username' ).val( '<?php echo $username ?>' ).attr( 'readonly', 'readonly' );
                } );
                </script>
                <?php
        }
} );

function get_unique_username( $prefix )
{
        $username = $prefix . get_random_string();
        if( username_exists( $username ) )
        {
                return get_unique_username( $prefix );
        }
        else
        {
                return $username;
        }
}

function get_random_string( $length = 5 )
{
        $characters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $charactersLength = strlen( $characters );
        $randomString = '';
        for ( $i = 0; $i < $length; $i++ )
        {
                $randomString .= $characters[rand( 0, $charactersLength - 1 )];
        }
        return strtolower( $randomString );
}