How do I send a booking for approval?

Nothing extra is required. If the booking violates the policy (mode "Show it and send for approval") or In-policy purchases is set to Send for approval, clicking "Book" automatically creates an approval request. The employee sees "Waiting for approval. You will be notified once it is decided." and can still change the pending trip in the basket, while approvers get the email "Trip approval required" and a notification, and decide on the Approval page (/app/approvals) under Trips → Active.