  Scattering resonances are an essential tool for controlling interactions of
ultracold atoms and molecules. However, conventional Feshbach scattering
resonances, which have been extensively studied in various platforms, are not
expected to exist in most ultracold polar molecules due to the fast loss that
occurs when two molecules approach at a close distance. Here, we demonstrate a
new type of scattering resonances that is universal for a wide range of polar
molecules. The so-called field-linked resonances occur in the scattering of
microwave-dressed molecules due to stable macroscopic tetramer states in the
intermolecular potential. We identify two resonances between ultracold
ground-state sodium-potassium molecules and use the microwave frequencies and
polarizations to tune the inelastic collision rate by three orders of
magnitude, from the unitary limit to well below the universal regime. The
field-linked resonance provides a tuning knob to independently control the
elastic contact interaction and the dipole-dipole interaction, which we observe
as a modification in the thermalization rate. Our result provides a general
strategy for resonant scattering between ultracold polar molecules, which paves
the way for realizing dipolar superfluids and molecular supersolids as well as
assembling ultracold polyatomic molecules.
