Prepopulate a field name based upon a zipcode table

Prepopulate a field name based upon a zipcode table

I have a created 2 forms, an application that people will enter their information.  Once they enter the field zip code I want to pre populate a field in form 1 with the city information.
 
Form 1 is the application
Form 2 is the zipcode with city and county fields
 
Pls help  I am new with building scripts