It is difficult to make specific suggestions without having any information about how fix external is used (there are two modes) and whether it is being called from C or from python.
But all its functionality is documented. There are tests in the unittest tree that showcase the use. Please see the discussion here:
https://matsci.org/t/lammps-users-running-lammps-with-forces-from-an-external-code/38228/4