// Display a contact form when the event is sold out
// to be used as a waiting list
function ee_espresso_clean_event_status( $event ) {
    $status = $event instanceof EE_Event ? $event->get_active_status() : 'inactive';
    return $status;
}
function ee_special_sold_out_message( $EVT_ID, $event ) {

    //Set the ID of the Ninja Form you wish to call here.
    $ninja_forms_id = 3;
    //Check if the event is sold out.
    if ( ee_espresso_clean_event_status( $event ) == 'DTS' ) {
        if( method_exists( 'Ninja_Forms', 'display') ) {
            //Using Ninja Forms v3+
            Ninja_Forms()->display( $ninja_forms_id );
        } elseif( function_exists( 'ninja_forms_display_form' ) ) {
            //Using a previous version of Ninja Forms
            ninja_forms_display_form( $ninja_forms_id );
        }
    }
}
add_action( 'AHEE__ticket_selector_chart__template__after_ticket_selector', 'ee_special_sold_out_message', 10, 2 );