How to add custom form submission (To other url) and validation instead of default submission form

Site URL: https://iconhealthco.com/new-member

I am trying to add the custom form validation and submit the form to third party URL, I am able to inject  the form submission code to another URL by the help of element.addEventListner but the form is now calling both events onSubmit. 

Is there any other ways to do the customize the form submission.?

Thanks in advance.