  We present precision spectroscopy on electrically trapped formaldehyde
(H$_2$CO), demonstrating key attributes which will enable molecular
spectroscopy with unprecedented precision. Our method makes use of a
microstructured electric trap with homogeneous fields in the trap center and
rotational transitions with minimal Stark broadening at a 'magic' offset
electric field. Using molecules cooled to the low millikelvin temperature
regime via optoelectrical Sisyphus cooling, we reduce Stark broadening on the
$J=5\leftarrow4$ ($K=3$) transition at 364 GHz to well below 1 kHz, observe
Doppler-limited linewidths down to 3.8 kHz, and determine the line position
with sub-kHz uncertainty. Our results and clear prospects for even narrower
spectra pave the way towards in-trap precision spectroscopy on diverse molecule
species.
