Thank you for your answer.
I'm concerned about filling up External MAC and Name fields, because it would counter the whole purpose of this functionality, if the tracker was able to connect to just one predefined device. It should be able to connect to various devices (drivers' phones) and report the actually connected device's MAC (who is driving).
Anyways, I tried to do it this way, and it didn't work neither, my User ID showed only zeroes.
After further tersting I accidentally found out, that the tracker actually reported the phone MAC, but it was only visible while bluetooth settings on the phone were open. As soon as the settings were closed, the reported ID turned to zeroes again. I suspect it is caused by some bluetooth visibility issues on the phone. I can't set it, to be always visible. Is there an Android app, that would automatically maintain the bluetooth connection when phone is in the bluetooth range of the tracker?