I want to wait for an API Call to complete before redirecting to another page on click of submit button in new_request_page.hbs .

But we want to do it programmatically... without using this option Modify the After Submit Behavior: 
in ticket forms.

The redirection should not happen until and unless a response is received from an external API call.

We are using like this:

 <div id="main-content" class="form">
        {{request_form wysiwyg=true}}
      </div>