I found it!!
Got to Settings → Configure Keyboard Shortcuts
There, search for “OpenGL Image Viewer”. And assign a shortcut for it.
It is really hidden to find it, the name is not clear at all.
Edit: I just noticed, that is not exactly the same as the “Show Fullscreen” command. The “Show Fullscreen” command actually calls the Slideshow, with the current selected image being the starting point. The OpenGL Image Viewer looks similar at first, but it does not have an option to play the images as a slide show, display the rating, etc. It will just show the plain image with no distractions. This is nice, but also possible with the Slideshow, I wish there was a shortcut to call the Slideshow directly from the current image.