Open popup after ninja forms submission - More on https://timersys.com/how-to-display-a-popup-after-user-submits-form/

functions.php

<?php // don't add this tag

// Print javascript code in the footer of the site to trigger the popup upon form submission
add_action('wp_footer', 'add_popups_trigger_code',150 );
function add_popups_trigger_code(){
    // show code on certain page, comment out to print code on every page
    if( ! is_page('520') )
        return;
?>
<script>  
// Create a new object for custom validation of a custom field.
var myPopupsSubmitController = Marionette.Object.extend( {
  
  initialize: function() {
    this.listenTo( Backbone.Radio.channel( 'forms' ), 'submit:response', this.actionSubmit );
  },

  actionSubmit: function( response ) {
    //insert popup ID here
    SPU.show('529');
  },

});

// On Document Ready...
jQuery( document ).ready( function( $ ) {

    // Instantiate our custom field's controller, defined above.
    new myPopupsSubmitController();
});
</script>  
<?php
}