Adds an approver to the invoice. Will select the first available approver slot that is not already filled and assign the approver to it. If no approver slots are available, an error will be returned. An explicit approver slot can be specified by setting the approverSlot field.
Invoice ID or Invoice ForeignID
The ID or the Foreign ID of the user to add to the approval slot.
The identifier for the approval slot this user is assigned to.