POP UP FUNCTION MODULES

Hi i am doing an enhancement in which(transaction va01) when the user changes the pricing conditions in the condtions tab then the user needs to be prompted to select a reason for the change. this reason codes are already there in the ztable that i have created!! so this popup should come that displays my Ztable to the user so that he can select any one reason for the change.

can u tell me what function module i have to use to do this!

moreover the value selected from the popup should go nad get saved in the vbak table.

plss help me.

points promised.