  A first observation is presented for the electroweak production of a W boson,
a photon, and two jets in proton-proton collisions. The W boson decays are
selected by requiring one identified electron or muon and an imbalance in
transverse momentum. The two jets are required to have a high dijet mass and a
large separation in pseudorapidity. The measurement is based on data collected
with the CMS detector at a center-of-mass energy of 13 TeV, corresponding to an
integrated luminosity of 35.9 fb$^{-1}$. The observed (expected) significance
for this process is 4.9 (4.6) standard deviations. After combining with
previously reported CMS results at 8 TeV, the observed (expected) significance
is 5.3 (4.8) standard deviations. The cross section for the electroweak
W$\gamma$jj production in a restricted fiducial region is measured as 20.4
$\pm$ 4.5 fb and the total cross section for W$\gamma$ production in
association with 2 jets in the same fiducial region is 108 $\pm$ 16 fb. All
results are in good agreement with recent theoretical predictions. Constraints
are placed on anomalous quartic gauge couplings in terms of dimension-8
effective field theory operators.
