Sales Order Customer Discount Update Upon Customer Select

Goal; to load up Customer Discount into Sales Order from Customer file. 

I created Discount items in the Item file.  In the Customer file I created a Customer Discount field ID: custentity_altp_cust_dscnt of type list/record soucing from the item file.  In Customer file the Customer ID is Field ID: entityid.  In the Sales Order Customer is Field ID: entity.  So I guess entity field information searches on entityid in customer file.  In the Sales Order the filed ID; discountitem will be updated from the field ID: custentity_altp_cust_dscnt

I am new to NetSuite Scripting and javascript.  I have a bunch of scripts to write so I figure after learing a few I will be able to make some progress.  My understanding is this is a PostSourcing script because the server is checked when the customer is selected.