Cannot create a Prepayment in OM Payments form

I believe all the setups have been completed to utilize the Prepayments process in OM.  Some of the documentation is very high level, though.

I am applying a payment line to the booked order.

Responsibility: OM Super User

Navigation:  From Order Actions>Payments

I enter the following:

Prepayment:  Yes

Payment Type: ACH

Percent: 100

Amount:  (generated by the amount entered in Percent, in this case 235 Euro)

Identifying Number:  ***This is the field that cannot be populated***

Receipt Method:  Prepayment - AR (auto populated based on the Payment Type setup)

Please advise.