Hi Kwee,
If it’s during the application registration that means the password that the user entered is still in the non-persistable entity of the registration, it means you can use a nanoflow, and inside of the call the sign-in activity from the Nanoflow Commons https://docs.mendix.com/appstore/modules/nanoflow-commons module from the marketplace, you need to pass it the registered user and the password entered by the user during his registration
Thanks,
Nour