Form - Reading a property of an entry made into the form

Hi All, my first post here. I'm new to aras and trying to figure out things.

I have added two fields to a form, a "Part Number" and a "Catalog Number". I have added a property "catalog_number" to the OOTB Part object.  In this new form, when the part number is entered by the user, I want to pull the catalog number associated to that part pulled into the catalog number field automatically. 

So, for example:

Part Number:  1234    Catalog Number: abcd

When the form is pulled by the item is is associated with, and the user picks part 1234 for the "Part Number" box, I want the "Catalog Number" box to pull the value "abcd" automatically.

Thanks for all your help.

Kamran