Tied mine on front axle, but will need something to secure back!
That looks a lot higher than the front axle? On the image it also looks like it is connected to the crashbars? Hope not as they will break easily.
If you used the front 'forks' (or whatever you call them on the bmw) take note that the straps might cause marks. Use something soft under them.
At least that nice trailer helps a lot to keep the bike upright in the front.
With that exhaust in the way you are going to struggle with a proper angle on the rear wheel straps. I normally kept the rear down and upright with straps to the subframe as in the image.
A wide enough angle on the straps and connected high up on the bike means low tension in the straps.