No, you can’t custom anything on this screen. This is strictly for authentication purposes (signing in with your email).
By “sign up”, I assume you mean the ability to let people configure their profile before they get access to the main content of the app. For that, you can follow this.
Yes, I have built the app as an onbiarding experience. Hiwever, Im affraid that for new users it will be not clear that they by “signing-in” they can actuality sign-up, if they dint have an account.
I don’t think that’s confusing if after signing in, they are immediately presented with a sole onboarding screen. Since they don’t see any other screens until they finish that onboarding step, that is clear that it’s the only thing they have to do in order to access the main content.
Users who sign out after finishing the onboarding process and sign in again later on would not be presented with the onboarding process again, so it looks like a good method to me.