Run Radiant and load up the map.

Select the door using shift+alt+click.

Press N.

Enter allowteams as a key and axis,cvops as the value and press return.  The axis door is obviously the more common, but you can make allied secure doors by putting allies instead of axis.  And if you don't put the cvops part in, then enemy cov ops in uniform cannot open the door.

Close the window and press ESC.

To indicate the secure nature of the door, we should put a new door texture on.

Select both faces of the door and apply Textures/doors door_m01asml_axis texture (if you are not sure which one this is, click on the texture you think it is, and check the name that comes up bottom right of the output window.)

Press S and Fit the textures (1 wide and 1 high).

Close the window, press ESC, save and compile.

You should find now that only Axis can open the door.  You can't test for cov ops alone so you have to ensure your spelling of axis,cvops is accurate.

Easy lesson!