The autoharp body is not very deep, so you don't have problems with the mike being inside the instrument like you do with guitars. They ar harder to experiment with on autoharp than guitars and other instruments, because they have to be placed in the soundhole, under the strings, to sound best, and this means you pretty much have to drill a hole and install a jack so that the wire does not hit the strings. Some people clip an omni mini-mike to the back of the harp, which sounds OK but not optimum, in my opinion. They use a piezo-electric crystal or a similar material that generates an electric current when it is vibrated. Magnetic pickups are loud, and great for bass response on an autoharp, while the piezo pickups have a more woody, natural sparkle.
Magnetic pickups have to go under the strings.
Most of them need to have a lot of midrange removed. Piezo pickups sound a lot better when they are used with a pre-amp. They put out a very weak signal, that can sound pretty awful if it's not connected to a device that is designed to accept those kinds of signals. Most amps and mixing board inputs are designed for magnetic pickups, which have very different electronic characteristics. To me, the autoharp is a very imperfect instrument, and this includes its sound.