On our old 23b it went above the center storage compartment in the slide. I think the main purpose is so the slide does dot start walking out during transit.
I wish I still had the trailer to take a picture and explain.
Here is a picture of the slide wen extended. When the slide comes in the transit bar went above the middle compartment. In the picture you can see a sticker.
So bring the slide in and place the bar in the oval opening. It is going to go between the slide and the main non moving part of the wall of the camper.
Hope this helps.