An idiosyncrasy of FileMaker is that it auto-enters the user name as the default account name in the credentials dialog.
User names are set in preferences. I wonder what you'd get if you used preferences to change the user name before generating the runtime solution?
I think it works, in that after re-setting Preferences > User name to 'User', and re-creating the RT, the RT opened without asking for a username at all.
Not necessarily. It may be accessing the user name set up in preferences the last time that you did.
Asking you for the user name the first time that you open it is part of the "self registration process". Try this with a completely different file name for your solution. I suspect that you'll still get asked for a user name. Might be a way to get an installer to do the needed update for a solution that you plan to distribute...
I saved the solution as another name, recreated the RT, all while preferences was set to User, and didn't get asked for username on opening the RT, so it appears to have worked.