Setting Dynamic Default to Current User on IR

Hi

I'm trying to create custom fields that store the user details for the person creating a purchase order and the associated item receipt.

I have created one that works fine for the PO: type List/Record based on employee and dynamic default set to current user. Store Value is ticked. Applied to purchases.

However I'm having less success with the IR. Can anyone advise how to set up the custom field so that the person creating the item receipt is automatically stored on the record? If I select Store Value then I can't get anything to display? If I untick Store Value it works fine when the record is created, however obviously the data is not stored and therefore changes when viewed by another user.... which defeats the purpose. I think the issue is something to do with the IR being associated with the PO. However the custom field is not applied to