How to incorporate captcha or turnstile into new user sign-up

I need to add a check for robots to my new user sign-up.

Currently on my sign-up page, I call app.emailPasswordAuth.registerUser(). I’ve added a CloudFlare turnstile to this page, which gives me token which I need to verify on the Atlas server using a POST request.

I can call a custom HTTP endpoint with the turnstile token, but if the token is verified, how can I then call registerUser() from the custom endpoint? Do I have access to app?

Alternatively, is there another way to make a POST request and only call registerUser() if that is successful? Going this in the client would defeat the purpose, as then a malicious user could call registerUser() without the check.

Thanks