Trying to understand the reasoning here.

"when users save that document for the first time they get a standard preference filename?? (They still should be able to change it into something they want)"

If the document IS saved, then that is now the name of the file. If the user "changes" it then:

1. there is an operation of SaveAs - which case there is now a NEW file with the new name;
2. there is an operating system operation that changes the filename.

I don't know what you have as a userFrom, but that is the way to go. I would:

Display, as a label, "File will be saved as..." followed by the filename YOU have chosen (by whatever logic). "Press OK to accept, or type in a new filename then press OK." They can either press OK - which accepts your name, or type in a new name and press OK.

OK simply saves the file by the name you have chosen, or if the texbox is not "", use that name.

Am I missing something?