How to update Shipments >> Salesperson (External Lookup) custom field?

How to update Shipments >> Salesperson (External Lookup) custom field?

Hi,

I am creating Shipment and passing Salesperson(Custom Field - External Lookup) data from Sales Order but it will not updating, if anyone have any idea then guide me!

FYI, I tried
1. customfield_id
2. api_name
3. label
but not worked any of above.

Thank You,
Himali K S