The next line after the get should be a READ

So your code would be constructed as follows:

@07,42 GET g_t0code ;
PICTURE "*I" ;
SIZE 1,10,1 ;
DEFAULT 0 ;
VALID back() or gett0()

READ

@07,42 say g_t0code picture '!!!!!!!!!!!!!!!'

If this code is within a window, or if the function called is within another window, you should activate the called window, do your thing there and deactivate it when you leave it, activating the first window in turn.

So your gett0 function would read:

activate window whatever
(code)
deactivate window whatever
activate window (from the calling snippet)