使用FormKeep，如何将请求的Accept标头设置为application / javascript

I'm using FormKeep https://formkeep.com/faq, and when I submit the form it redirects to a thank you page. But I want it to stay on the page and submit via AJAX. Below is my code, but in order to disable the redirect, they say I have to set the Accept header of the request to application/javascript., an