How do you create a pick list field that auto fills another field?

How do you create a pick list field that auto fills another field?

I am new to this and am trying to do a form in zoho by remaking the purchase order form to be a shipping form.  Basically I want to send shipping information from an account to a shipper for them to pickup and ship my items. I cannot for the life of me figure out how to set up an "auto fill" feature in this form.  Please help.