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I got a "LIVALL® P1 Smart Bicycle/Bike Nano Cadence Sensor", round and flat which appears to have a screw in for the end of the crank. However on all the bicycles I see there is the hexagonal screw nut and a black thing around it... just about the size of the threads.
Is this compatible with some way to remove that black spacer or do I need to try to awkwardly zip tie it to the crank somewhere to not rub on anything?
from Maker's Website
Looking at the video (the English version I linked in the comments), there are several options for installation. The sensor can act as a replacement for the dust cap found on many cranksets (methods 1 and 2). The dust cap sometime screws in. This doesn't work if the dust cap and crank bolt are combined (as on Octalink, and similar to what you describe). I don't think the sensor is designed to replace the crank bolt in that case, but even if it is, I'd be very cautious. Instead the 3rd option (cable-tied to the inside of the crank arm) is probably the best bet
The black spacer you see is either a plastic dust cover or crank extractor. If there are small holes in it, it is a crank extractor and you can use a pin spanner to unscrew it. If not, it is just a plastic dust cover and you can pry it out.
It should be noted that there are many crank designs, and it is the three piece square taper one that has the dust cover. It used to be most common until recently, but all bikes do not have it.
