Ship-to Location on the Purchase Order form to default to null

Our requirement is to make the Ship-to Location on the Purchase Order form to default to  null for few suppliers irrespective of the values on the  supplier setups. I tried doing this Using form personalization.

Here are few Scenarios: When I enter supplier Name and tab out , the cursor moves to Site field. So I used WHEN-NEW-ITEM-INSTANCE on  PO_HEADERS.VENDOR_SITE_CODE and set the property of  the PO_HEADERS.SHIP_TO_LOCATION as Update_null true. This works fine and ship_to_location is null.

But When enter supplier and manually place the cursor on any other fields other than SITE the ship_to_location populates from the supplier setups. .