Authenticate user password in flow

Hi,

As one of the steps in a flow, I would like to force the user to (re)authenticate with his Sharepoint Online password. If he/she typed the correct password then he/she can progress to the next step.

The authentication could be done by:

   A) User types the password on a custom form and then press "OK" to authenticate. (Ideally)

   B) User is prompted to login to Sharepoint (again) thus authenticating. (Less optimal solution) 

Can you point me in the right direction on how to build this? Any existing solutions? 

All help will be very much appreciated! 🙂