It’s similar to what I do here. You can’t add anything to the cart until you sign in. I use the same button but with a custom action that forces you to sign in. Once signed in, then the button will allow you to add to cart.
I don’t remember, but I may have something in there to pass in details from the user profile, or let the user change them while completing their order. I don’t remember if I did anything to actually save updated values back to the user profile.
So my example may or may not contain pieces of both scenarios.