autofill a field, based on a lookup drop-down list

autofill a field, based on a lookup drop-down list

active form is <test>
in <test> there is a lookup field <Item_Num> which is a dropdown lookup
field to autopopulate in <test> is called <Item Description> which is just defined as a text field


lookup form is <Item_Master>
this form has field <Item> that is used to link the two forms meaning <Item_Master.Item> is linked to <Test.ItemNum>


In <test> in the field <ItemNum> under the "On User Input" section I have the following script:
[note that I am new to script so am I missing headers, "{" or other lines??]


ID == input.ItemNum ;
rec = Item_Master [Item = ID] ;
input.Nii_Item_Descrip = rec.Description;


why does this not work?
thank you