How to create/assign functions from Home that are like F1, F2 of Function app

[MigPrime]

I'm a fairly new user and can't figure this out: HP Prime G2
The Function app has several function variables, F0 ... F10 that can be edited in the symbolic view which is obviously the easiest way.
However, F1 etc can be manually assigned to any *expression* in the Home view on the command line like this (hinted at on p 510 of the Prime Users Guide :
"For example, you can store the expression “SIN(X)” in the Function app variable F1
in Home View, or you could open the Function app, navigate to F1(X), and enter SIN(X) in that field.
F3 := "sin(x) .... "
(either single or double quotes work)
and the result shows up in the Symbolic view of Function and appears to be exactly equivalent to entering it on the Symbolic page.

My question: is it possible in Home view on the command line to similarly assign/define a user function variable (similar to F1 ..) to an expression?
For example, I tried this but it doesn't seem to work:
Tnew := "cos(x) ..."

(Using shift+Define to create a user function doesn't seem to produce a variable which is like the F1, F2 ... variables of Function app.)

So how does one create variables that behave like the F1 ... variables of the Function app?

Thanks for any suggestions and explanations.