Select Render Walk → Measurements from the side menu.

Select the room you want to add measurements to and click the "+" button on the top right.

Measurements - select room and click + button

Now drag the points to indicate the measured distance, then click Save.

Now you can modify the appearance of the drawn measurement, add a label, and change the size, shape and color of each point and line.

Click Save.

You can view a live preview of measurements in this demo.