not sure if I got this right...
the user can be set in settings (first tab, section 'username') - by default, it's the OS user. Can be changed. There should be no difference between local/remote files, it's a FileMaker setting, not a file setting
Yes, I know, but this way you can choose an user (and you have to add password, too) for each filemaker database.
If I try to open a different file, the problem persist, and I would add user and password for each one.
More than this, I cannot add only username, and if I choose to add username and password in a file, I enter directly without typing password. I don't like it too much, even if I don't share my MAC and I have protection when left more than 2 minutes "alone".
I imagine filemaker takes this username (standard is the same of MAC OSX) from some MAC OSX library, I'd like to store a different one, but probably it is not possibile.
This is in the Prefences setting.
With or without a file open, goto Edit>Preferences and over type the User Name box on the General tab. Then hit OK.
Once done, whenever you open a file the new user name you typed above will be, by default, set into the user name section when opening a file. Then simply enter your password.
You can get FileMaker to auto log you in by setting the "Allow Credential Manager to save password" if you also enter the "Log in Using" settings on File> File Option menu on an open database.