Probably just tie-wrapped to a chassis rail. usually near the front of the car fairly low down. the transponder prefers a clear view down to the track.
If your hiring it it will be battery powered and self contained.
It's about the size of a fag packet, so find yourself a preferred location, and a range of mounting options (various lengths of tieiwrap, worst case a large Jubilee clip) If you're lucky it will include a plastic bracket with tie-wrap holes. The transponder mounts with an R-clip