Menu default appliication

I need to reference external (static) page inside MENU application. I tried to link blank App and place the code in there. It worked locall but not on the production server.
I then tried to reference instead URL, but this doesn’t work as well. I simply get error 404 (page not found).

The Menu App is a vertical (responsive) menu and I want to fill the rest of the screen with some custom HTML.

Not sure how the referenced path should work. Could anybody post some hints, please…