how to populate billto_addressid

Can someone tell me how to populate billto_addressid

I select the Lookup address and select Bill To Address and then select an address in the drop down list.

It does populate the actual address on the form, but I actually need the billto_addressid to be populated.

I am assuming I have not configured something as this field is always blank, but in our old CRM, this value was always populated.

Help please.