F4 help for one field in modulepool program..(Possible values)

Hello Experts,

I have 3 fields in my module pool screen.

PLANT, MATERIALNO AND STORAGELOCATION.

BASED ON PLANT AND MATERIAL I NEED TO PROVIDE F4 HELP FOR STORAGE LOCATION(LGORT).

How can i write the code in PROCESS ON VALUE REQUEST ..

AND WHICH FUNCTION MODULE SHOULD BE USED.

Please can anybody send me the code asap.

I will give u full points if ur code matches my requirement.