This is an example of how to add a Ninja forms form to an event that has any of the ticket types within it sold out.

example.php

<?php //Please do not include the opening PHP tag if you already have one  
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file
// Display a contact form when the event is sold out
// to be used as a waiting list
function ee_espresso_any_sold_out_tickets( $event ) {
    $tickets = $event instanceof EE_Event ? $event->tickets() : null;
    
    if($tickets) {
        foreach($tickets as $ticket) {
            if($ticket->ticket_status() == EE_Ticket::sold_out){
                return true;
            }
        }
    }
    return false;
}
function ee_special_sold_out_message( $EVT_ID, $event ) {
    
    //Set the ID of the Ninja Form you wish to call here.
    $ninja_forms_id = 1;
    //Check if the event is sold out.
    if ( ee_espresso_any_sold_out_tickets( $event ) ) {
        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 );