Are you sure that will work with MotionEyeOS? We are not using apache. In fact, I can't figure out what web server it's using, it seems to be homemade. By inspecting the webpage, I found that the web root is /usr/lib/python2.7/site-packages/motioneye. But if I place a test.html file in there, it cannot load in a browser, I get
{"error": "not found"}
So that makes me think .htaccess would not work either.
There is already an option in the MotionEye GUI for Digest or Basic authentication, but it's not documented.