Yes, you have to pull the tank, to run the cable there. How to do that, is described in the service manual.
Here is a video, which describes, how to lift the fuel tank:
Undoing the fuel line under the tank is a little bit fiddly.
The company, where I bought the mount is `Louis'. They have an English language homepage, too:
https://www.louis.de/en
I also edited the link in my origin posting, to warp you to the English version of the mount product description.
The power supply cable coming from the Garmin socket is open ended, ready for getting crimped somewhere to the internal power supply of the CB. But you could just install a plug at this end, which fits into your battery's pigtale.