Get the user to accept Terms & Conditions when signing up with OAuth

The register form I’m working on allows the user to sign up using their Facebook account or to create a new account by typing their email and password. Here a mockup:

Currently, clicking on the Facebook button (and authenticating via the Facebook pop-up) redirects them automatically to the next page in the sign up flow. However, this means that the user can sign up without explicitly accepting the terms and conditions of the page.

I’m looking for the best way to “force” the user to accept the terms and conditions when using OAuth. Some options I thought about:

• Clicking on the “Use Facebook” button marks the Terms checkbox as invalid (if unchecked) and doesn’t proceed unless it’s checked — feels awkward because the checkbox visually belongs to the fields that can be skipped when using OAuth (email, password)
• Add a second Terms checkbox right below the Facebook button and use the same idea — having two checkboxes on the page seems unnecessary
• After successful login via Facebook, replace the Email and Password fields with some information about the Facebook account of the user and let them register using the Sign Up bottom on the bottom of the page (and validate the checkbox) — increases the actions needed to create an account and would require big changes in our backend
• Show a pop-up asking the user to accept the Terms and then proceed to Facebook authentication — probably the best idea so far, but I’d like to avoid using multiple pop-ups

Does anyone have a better idea? Unfortunately we can’t use a generic “By signing up you automatically accept our terms …” text because of GDPR (the user has to consent manually).