Transferring migration network password from Windows to Mac Pro

when windows and macOS computers are involved, each computer is running Migration assistant.

to access the files on the SOURCE computer, you need to provide credentials equivalent to what you would need to log on to that computer and read all the files you want to copy. Except the dialog appears in a window on the Mac.

It's not a secret code. It's what is at the username and password for the Windows computer, so that its files can be read.

... and the Windows computer needs the correct version of Migration Assistant running.

Start with this article.

Transfer from PC to Mac with Migration Assistant - Apple Support

If that does not answer you questions, please Provide MUCH more information about exactly what you are trying to do.