Sitecore third party form inside the sitecore form

Looking for a way to incorporate a third party form (not a sitecore form) into a sitecore layout.

I suspect it is not working because of the outer sitecore form tag (the one that wraps all content). Problems due to nested form tags (not good).

<div><form id="non-sitecore" action="" method="POST">
  <input ...>...

What is the best way to get this type of third party form to work?