Hi!
Welcome to Microsoft Q&A forum.
What your MAC is asking for is not your MAC name, it wants the login credentials of a user account on your Windows PC.
First, the username of your WindowsPC like: john, adminuser or you sometimes your email. (that display user upon login)
Next, the password you use going to the desktop (if none you need to set one and make sure file sharing is enabled)
Try it first then let me know once you tested it so I can assist you further.