USER DEFINED F1 documentation for seelction screen fields.

I have one selection screen in my Program.

If I give F1 , It has to display user defined documentation.

like...I have customer field in selection screen.

Select-options:S_CUST for BSAD-KUNNR.

For this I need doc as "Enter customer number excluding from the Table"

Not the documentation of the KUNNR field.

How to do this.

Many Thanks!!!