Leadin should work right out of the box with NInja Forms, no special integration required.
Leadin should work with most WordPress form plugins and custom built HTML forms. The only exceptions are forms that are not enclosed in a HTML <form> tag or if the form is loaded through an iFrame. If the form submits through Javascript we can’t detect that it’s a form, or if it’s an iFrame we can’t detect the submission because of cross site security reasons.
Let me know if run into any problems and I’ll be happy to help. I’m going to mark this ticket as closed for now.