Well, it doesn't matter what resolution you run your program in, what matters is if the windowed mode is on or not. If you were to put this command "Set Window On" in your code the program would run in windowed mode. If you don't use that command then your program will automatically run in fullscreen mode regardless of the resolution.
But what I think your asking is how do you print text right in the middle of the screen, right? Thats what the "Center Text" command does.
This from the help files:
CENTER TEXT
This command will output the provided string using the current text settings at the specified coordinates. The text will be centered on the X coordinate given. The coordinates should be integer values.
SYNTAX
CENTER TEXT X,Y,String
Let me know if that helps or not
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