credit invoice workflow

We would like to route credit invoices in Billing through the Approval Framework delivered process.  We have modified the business unit, created the approval definition and defined the user list.  Then we created a credit invoice, set it to RDY, and ran the Billing Workflow process, but it doesn't appear to be submitted for approval.

Other approvals have a Submit button somewhere that is used to submit the transaction for approval, but there doesn't seem to be one in Billing.  How do we submit credit invoices for approval?

Thanks,

Andy