Click the Openings button
Choose the opening type from the library.
Click on the opening and drag it to a wall.
The opening will connect to the wall.
Click on the center of the opening to access the measurement tool.
Enter a measurement and click the green check mark to confirm.
You can also drag the opening to the correct length by tapping on one of the ends and adjusting it to the correct length.
The measurement will auto update as you drag the opening.