Concidering webflow, cant see information about integration with custom code

I am considering moving/redesigning our website in web flow. I am only interested in doing the public-facing pages (not our dashboard or app pages). We have a custom domain.

As well as the usual landing/pricing/about etc pages, I will need a signup page, a login page, a reset email page etc. I can see how easy it is to make them, however I can’t see anywhere in the tutorials how I would plug them in to get them to work.

If I use our current signup page as an example. We have a JS file with all the relevant scripts in, so that on click of the signup button we validate the inputs and then run some custom api to create an account and then show a thank you message (currently on the same page but could be a redirect).

To be clear, we already have a Db and everything in place, I’m not asking how to do this from scratch and I don’t want any 3rd party integration to help manage this. I essentially just want to use the same code as before is this possible?