1. Select the Contours layer in the Contents window.
2. Click the Levels tab in the Properties window.
3. Set the Level method to Advanced, and click the Edit levels button.
4. In the Levels for Map dialog, click the current level that will follow your desired contour.
5. Click the Add button. The new contour level will be added directly before the level selected in Step 4.
6. Double click the new contour level value, enter the desired value in the Z Level dialog, and click OK.
7. If desired, adjust the Line, Fill, Label, and Hachure properties.
8. Click OK. The map will now show the newly added contour line.

Updated October 20, 2021