How to default the dynamic value on F4 Dialog(Value help dialog)

Hello Experts,

AS-IS:

i have a "Region" field on screen, after enering any value in 'Region' input box i am pressing enter and then storing value into Parameter ID.

in the Search help exit i am reading parameter ID and setting as a default value. (This will work only when the user press enter).

Problem here is some times user Just entered some text in the "Region" Field, and he forgot to Press enter key.

and directly pressed F4 key on the city field ( country is taking the old one if not cleared).

Thanks in Advance..

Regards,

Kishore