  A measurement is presented of the cross section for electroweak production of
a Z boson and a photon in association with two jets (Z$\gamma$jj) in
proton-proton collisions. The Z boson candidates are selected through their
decay into a pair of electrons or muons. The process of interest, electroweak
Z$\gamma$jj production, is isolated by selecting events with a large dijet mass
and a large pseudorapidity gap between the two jets. The measurement is based
on data collected at the CMS experiment at $\sqrt{s} =$ 13 TeV, corresponding
to an integrated luminosity of 35.9 fb$^{-1}$. The observed significance of the
signal is 3.9 standard deviations, where a significance of 5.2 standard
deviations is expected in the standard model. These results are combined with
published results by CMS at $\sqrt{s} =$ 8 TeV, which leads to observed and
expected respective significances of 4.7 and 5.5 standard deviations. From the
13 TeV data, a value is obtained for the signal strength of electroweak
Z$\gamma$jj production and bounds are given on quartic vector boson
interactions in the framework of dimension-eight effective field theory
operators.
