F1() evaluation in RPN mode?

[tppytel]

I feel like I've resolved this before and then forgot...

In Home/Textbook mode, I can define a function in the Function app, then do (e.g.) F1(5) to evaluate the function at x=5.

How do I do this in Home/RPN mode? I read that I want to put 5 on the stack and then F1(1) to indicate a single argument, but this just gets me a query about defining F1 as a variable. Can't Home/RPN read F1 from the Function app directly like Home/Textbook can?