  We introduce a theoretical and computational method to design resonant
objects, such as nanoantennas or meta-atoms, exhibiting tailored multipolar
responses. In contrast with common approaches that rely on a multipolar
analysis of the scattering response of an object upon specific excitations, we
propose to engineer the \textit{intrinsic} (i.e., excitation-independent)
multipolar content and spectral characteristics of the natural resonances -- or
quasinormal modes -- of the object. A rigorous numerical approach for the
multipolar decomposition of resonances at complex frequencies is presented,
along with an analytical model conveying a direct physical insight into the
multipole moments induced in the resonator. Our design strategy is illustrated
by designing a subwavelength optical resonator exhibiting a Janus resonance
that provides side-dependent coupling to waveguides over the full linewidth of
the resonance and on a wide angular range for linearly-polarized incident
planewaves. The method applies to all kinds of waves and may open new
perspectives for subwavelength-scale manipulation of scattering and emission.
