Wordpress custom form post to different page

I have a site which is developed with PHP and recently I added Wordpress blog(same domain with /blog url) to that.

I would like to implement two extra functions(subscribe/contactus) on that blog which is already existing & handling on PHP site.

I found few plugins like Visual Form Builder to build custom forms and also widgets.

I need to use existing php back-end functions to handle contact-us and subscription functionality in the blog as well. My question is how to change the action url on those custom build forms(in the blog) to post details directly to that existing functions in the PHP site.

Note : i'm very new to wordpress.