Integrate PHP Form in Upfront - Fixer

I’m planning to develop my own PHP form.
Within WordPress, installed with Genesis, I can set my PHP form in the Templates directory, then create a page, assign my PHP file as template for that page and everything works fine, even the layout.
I’d like to do something similar within a WordPress website using UpFront. How can I do that?
I guess my question is: How can I develop my own PHP form for a web page and make sure that it can be integrated within the layout and UpFront.

Thanks and kind regards,
Max Mesman