Dialog Programming: Screen Parameter ID

I have a developed a screen with the following fields:

Material:

Plant:

Sloc:

For the Sloc, I want a picklist to show up for all slocs for the Plant the user entered. Currently I can only get it to display all slocs using the parameter ID LAG.

How can I get this to work? If it involves code, can you please include an example and where I should put it? thx