You can assign a password to a user in a microflow. Retrieve user: change object, select attribute.
However, this is not the recommended way to work with passwords. A better approach can be:
Advantages:
Disadvantage
Using one generic password is definitely not a good idea. I fully agree on Chris' solution, though you could consider another solution which might be easier to implement (no deeplinks).
Again, doing it with deeplinks is more secure. This is another option should you be unable to get the deeplinks to work (being a newbie and all) :)
Good luck!