How to default Bill to BU on Purchase Order for shared service

Summary:

How to default Bill to BU on Purchase Order for shared services setup (Payables, Payments) without creating different sites.

Content (please ensure you mask any confidential information):

Hi Gurus-

We have Req BU = B1 , PRC BU = B1 and Payables BU = B2 , B3

So when we raise a Purchase order under B1 the business can pay it either B2 or B3. And to do this Bill to BU needs to be on the right BU (B2 or B3) as this connects with the accounting details as it goes to Payables.

But the only way we can see how the bill to BU can update/default to the right value is