The best is to take a look at one of the existing aircraft, for example the Cessna.
To make it appear among the aircraft, your objdef file has to have "physics" : "jsbsim/...". The Cessna has "jsbsim/c172r" there; the c172r is the name of JSBSim FDM model found in jsbsim/aircraft/c172r
Then you also need the aircraft script file that links the aircraft to the inputs, controls the animations etc.