I don't believe there is a straight forward way of "getting credentials" from Windows in PAD. That would be pretty bad honestly, since you would then easily be able to deploy bots that forward these informations while pretending to perform actual tasks.
If you are not using a shared login, the individual users of the flows would have to edit them and enter their credentials, as far as I'm aware. A guide could possibly be made for this, to assist them.
Alternatively, if you are afraid the interface is to complicated for them, you could incorporate a step where they initially fill out their credentials and save them somewhere (either locally, in Dataverse or somewhere else) and afterwards just get them for this location. But this again would be a reduction of safety.
If you can, maybe reach out to the application developer and ask them to implement SSO - this could circumvent the problem.
Feel free to inquire, if you need more information or have supplementing questions.