Open popup after WpForms submission - more on https://timersys.com/how-to-display-a-popup-after-user-submits-form/

functions.php

<?php // don' add this tag

// Open popup after WpForms submission
add_action( 'wpforms_process_complete_534', 'spu_popup_trigger' );
function spu_popup_trigger(){
  add_action('wp_footer','spu_open_popup',99);
}
function spu_open_popup(){
  echo '<script>setTimeout(function(){SPU.show(529)},1000);</script>';
}