I’m not sure what your question is, but the TMS example provided will analytically calculate the magnetic field from a coil (modeled with dipoles) in the  SimulateForwardMagnetic, then convert them to current sources which are used as know in the FEM potential calculation.  You can calculate the potentials at detector positions by extracting the potentials at those locations.  You could generate a type of leadfield matrix by doing this calculation for a delta function with all sources and concatenating the results.

cheers,

Jess