Inserting a variable into a dynamic text field

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I know you can read the contents of a text file into a dynamic text field at runtime. Can you also insert the string contents of a variable into a dynamic text field at runtime? I simply want a dynamic text field that says “Welcome back, <username>” I am doing it right now with a static text field and a dynamic one, but if the username is really short, the whole thing is not centered and looks weird. Optimally, it should all be part of one element on the stage. I can’t think of a solution though.