How can I make a Sign up page which pre-populate some values?

I have two domains

root domain: wordpress site

& sub domain: glideapp site

Now, I want to build glide app on and want to integrate a sign up page where user can register to app. Above that, I want to provide a refferal program where user can enter the referral code to unlock the reward they receive once they get login access. Also, I want every user to generate a link and code to share with other users to join. Basically, same refferal mechanism like dropbox has. In my case It will be Give $5 and Get $5.

So I have three problems to solve:

  1. Building a sign up form where user can register to app to get login access. What is the best method?
  2. Can I build some sort of functionality where users can sign up via refferal link and the code gets auto populated in the form at the time of sign up.
  3. Also is there a way, that code will store in a cookie and can transfer between root and subdomain and then that code gets auto populated at the time of sign up?

Looking for an expert advise. Thank you so much!