Hi,
When you create TMG,In background It creates module pool program.
So in SE80 , select FUNCTION GROUP and write name of FUNCTION GROUP which you gave when you created TMG, after that in SCREEN FOLDER click LAYOUT OF SCREEN
you will be able to see field of TABLE ,click on value columns of your field edit screen
and than in ATTRIBUTES go to PROGRAM and select value in drop down NOT POSSIBLE.
Let me concern If you find any issue, following above steps.
Regards,
Dular Sharma.