This is can be done as follow:
After the user fill the registration form, then login him in, then take him to the same page as fido2 registration to follow the procedure, you can save the base_usernmae cookie to make him login quickly next time.
We are doing something similar by using google login form idea, where the user enters the username and if there is a 2FA, get directly to it without entering password.
For further actions, you may consider blocking this person and/or reporting abuse
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This is can be done as follow:
After the user fill the registration form, then login him in, then take him to the same page as fido2 registration to follow the procedure, you can save the base_usernmae cookie to make him login quickly next time.
We are doing something similar by using google login form idea, where the user enters the username and if there is a 2FA, get directly to it without entering password.