  In the vanilla type-I seesaw leptogenesis scenario, CP violation required to
generate the lepton asymmetries in the heavy Majorana neutrino decays stem from
complex Dirac-type Yukawa couplings. In this paper we explore the case in which
that CP violation originates from the vacuum expectation value of a complex
scalar singlet at a very high scale. This non-trivial CP-violating phase can be
successfully communicated to the low-energy neutrino sector via the heavy
neutrino portal. The new scalar-singlet degrees of freedom generate new
contributions to the CP asymmetries relevant for leptogenesis not only at the
one-loop level but also through tree-level three-body decays. These are
computed here for an arbitrary number of heavy neutrinos, Higgs doublets and
scalar singlets. We also take into account the new decays and scattering
processes that enter the unflavoured Boltzmann equations governing the
heavy-neutrino particle densities and the $(B-L)$-asymmetry evolution. Having
established the framework of interest, we present a simple model with two RH
neutrinos, two Higgs doublets and a complex scalar singlet, supplemented with a
$\mathcal{Z}_8$ flavour symmetry. This symmetry minimises the number of free
parameters without compromising the possibility of spontaneous CP violation and
compatibility with neutrino data. In fact, the only viable $\mathcal{Z}_8$
charge assignment shows a preference for a non-trivial spontaneous CP-violating
phase, which in turn leads to a non-vanishing CP asymmetry due to the direct
link between high- and low-energy CP violation. An interesting feature of this
simple setup is that the usual wave and vertex type-I seesaw contributions to
the CP asymmetry vanish due to the $\mathcal{Z}_8$ symmetry. Thus, the observed
baryon-to-photon ratio can be explained thanks to the new couplings among the
heavy neutrinos and the new scalar degrees of freedom.
