How can I define my own F4 Input help processing for a field on the selection screen?
You can use the event AT SELECTION-SCREEN ON VALUE REQUEST FOR <fieldname> for defining your own input help for selection screen fields.
How can I set the initial values for SELECT OPTIONS at the start of the program?
In the event INITIALIZATION and AT SELECTION-SCREEN OUTPUT you could set the initial values for the selection screen fields. For SELECT OPTIONS you should fill the SIGN, OPTION, LOW, HIGH fields.
Eg: Set the date field with starting day of the month to last day of the month.
How can we change screen fields on run time?
The parameters and radio button selection at Selection screen can be changed as below...