Thats the Spokey Joe trailer which, as you can see, mounts to the rear axle.
But theres a problem. The Somerby is a rear hub motor, and for its physical size these motors produce a lot of torque. To manage tbis the various brands have different types of axle profile that lock into the droputs, and at the clamped up tight on each side by a nut.
The danger is that by putting something else, in this case the trailers towing hitch, under one of those nuts has the potential to cause problems as it adds an additional interface and quite likely reduces the clamping efficiency. This could allow the motor to move, and should it do so runs the risk of damage to motor, frame, or, more probably, both, notmto mention the risk of an accident.
Physically that trailer will possibly bolt on, but I would suggest it is highly inadvisable.