gistfile1.txt

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 = $prefix . rand( 1000 , 9999 ); 
                ?>
                <script type="text/javascript">
                jQuery( function($) {
                        $( '#username' ).val( '<?php echo $username ?>' ).attr( 'readonly', 'readonly' );
                } );
                </script>
                <?php
        }
} );