What i understand from the code is that - (correct me if i am wrong)
As the FILL=C'0' in specified for the input area, if no value is entered on the screen and ENTER is presed, zero will go to the program.
But objective is to show the default value when the scren is thrown for the first time.
I would like to state another example in this regard,
When the screen is thrown for the first time this will be displayed:
ENTER EMAIL: firstname.lastname@example.org
If i update the field it will become:
ENTER EMAIL: email@example.com
So the UNDERSCORE is the default value for that input field and also is updatable.