There is actually no way to manage it in the sign-up policy...
We have created samples to do this in custom policies here:
While we realize this is only works for custom policies (the part where you can track versions of consent), we currently don’t have plans to implement this in built in policies.
Gregor Wolf commented
It's interesting that the documentation at https://docs.microsoft.com/en-us/azure/active-directory-b2c/manage-user-access#capture-terms-of-use-agreement mentions the support for built-in policies. But I can't find it when editing i.e. "Sign up and sign in"
Hrvoje Kusulja commented
I also confirm, that according to EU GDPR; users needs to select checkbox and mark as "I agree" on terms. This action also needs to be logged. It must be prior or during the registration process on AAD B2C. So what about this feature?
We need to get our users to agree to the usage of their details when they sign up. This is for compliance with the General Data Protection Regulations that come into force in the EU in May 2018.
This would have to apply to signing up with social accounts as well as signing up with a local account.