Congrats on buying a ST2 comfort. Your options for carrying a Ulock may be limited to a rear trunk bag like an Ortlieb trunk bag with a Racktime adapter assuming the Ulock fits inside the bag (not sure what your dimensions are). Whether or not you want to carry a lock this way, the Racktime adapter for rear trunk bags can be fitted to the narrow rear carrier that comes with your Stromer by following the directions for the adapter on how to disassemble it and invert the two "straws" that sit inside the top of the carrier. Or, if you don't like this solution, you can carry the lock in a backpack.
Alternatively, many Stromer owners have bought the Abus Bordo Granit X-Plus 6500 foldable lock that can be mounted to the water bottle bosses using two metric stainless steel button head cap screws, size M5-0.80 x 20, but you will also need nylon washers to use as spacers between the frame and the lock's cage because the Abus Bordo cage has curved ridges designed to mount to a round frame, and the Stromer downtube is relatively flat. The 6500 comes in white or black. If you want a pair with the same key, xxcycles.com sells the 6500 in pairs if you don't mind the international shipping. Some of the pictures you will see of Stromers posted by European owners show the 6500 mounted to the water bottle bosses. The right metric threading for the water bottle mount screws (the originals on your Stromer are too short for the 6500) is critical or you risk stripping the threads, so be careful you buy the right ones. Also, since the screws are going into the battery compartment, they can't be too long.
