You mean you want your regular HTML5 game to be locked to landscape view on mobile devices? I don't think it's possible to force this in browsers.
However there is a setting "Orientations" in the project properties. You can set it there, but it does also say "if this can be enforced". Best give it a try.
What you can definitely do is check for the orientation with the browser object and display a warning message if it's in the wrong one.