The intrusctions below go through the steps on adding shipment(s) to an existing trip
Navigate to the Dispatch tab on the left hand side bar
Click on Long Distance on the very top of the page
Ensure you're on the Shipments tab
Select all the shipments you would like to add to a trip
Click on the Actions dropdown menu and select Assign to Trip
Viewing the Modified Trip
To view the trip that was just modified, click on trips & search for the desired trip. You should now see the added shipment as part of the trip.