  For a Hermitian Lie group $G$ of tube type we find the contribution of the
holomorphic discrete series to the Plancherel decomposition of the Whittaker
space $L^2(G/N,\psi)$, where $N$ is the unipotent radical of the Siegel
parabolic subgroup and $\psi$ is a certain non-degenerate unitary character on
$N$. The holomorphic discrete series embeddings are constructed in terms of
generalized Whittaker vectors for which we find explicit formulas in the
bounded domain realization, the tube domain realization and the $L^2$-model of
the holomorphic discrete series. Although $L^2(G/N,\psi)$ does not have finite
multiplicities in general, the holomorphic discrete series contribution does.
  Moreover, we obtain an explicit formula for the formal dimensions of the
holomorphic discrete series embeddings, and we interpret the holomorphic
discrete series contribution to $L^2(G/N,\psi)$ as boundary values of
holomorphic functions on a domain $\Xi$ in a complexification $G_{\mathbb{C}}$
of $G$ forming a Hardy type space $\mathcal{H}_2(\Xi,\psi)$.
