Membership management w/o post signup URL

I'm creating a webapp that lets creators use most of the features except premium ones require a paid membership. Not being fluent in backend development I'm leaning towards using a membership management tool such as Outseta or Memberstack.

After watching a few videos on the Outseta website, I saw that the they require a post signup URL, which would potentially erase the content so far created.

My ideal solution would be an in page js modal in which the user signs up and pays for the membership, then goes back to where he left, with now having the premium features available.

Any suggestions?

Thanks in advance.