OM Milestone Billing: Where/How can we manually enter the completion dates for custom milestones?

We have a business case for Milestone Billings and Doc ID 2611243.1 provide good documentation on the setups and process flow. However, it does not provide the navigation and steps for manually entering completion dates for custom milestones.

Doc ID 2611243.1 states that: “Invoices for custom milestones such as Delivery, Installation, etc. are automatically generated based on the completion date. Completion dates can be entered manually or through open interfaces.”

 Where/how can we manually enter the Completion dates for custom milestones?

Please see attached steps with screenshots.