Let $G$ be a connected complex reductive group with a maximal compact subgroup $K$.
Let $\lambda$ be a dominant weight in the interior of the positive Weyl chamber. Let $V\_\lambda$ denote the irreducible representation with highest weight $\lambda$ and fix a highest weight vector $v\_\lambda$. Then $gB \mapsto [g. v\_\lambda]$ 
gives a $G$-equivariant embedding of the flag variety $G/B$ into the projective space $\mathbb{P}(V\_\lambda)$. 


Fix a $K$-invariant Hermitian product on $V\_\lambda$. This defines a $K$-invariant Fubini-Study form on $\mathbb{P}(V\_\lambda)$ and hence on the image of $G/B$ in there. 


My question is whether this symplectic form on $G/B$ coincides (up to a scalar) with the Kostant-Kirillov form on the coadjoint orbit of $\lambda^\*$ (dual weight)? (via the moment map $\mu: \mathbb{P}(V\_\lambda) \to Lie(K)^\*$) restricted to the image of $G/B$.)



Kostant-Kirillov form versus Fubini-Study form on Plucker embedding
The answer is yes, due to formal properties of moment maps (that depend on almost no details of your situation). Namely, suppose $K$ acts transitively on any symplectic manifold $(X,\omega\_X)$, with *equivariant* moment map $\mu:X\to\mathfrak k^\*$. Then $\mu(X)$ is a coadjoint orbit $\mathcal O$ and $\omega\_X=\mu^\*\omega\_{\mathcal O}$.


Proof: Write $Z(x)$ for the infinitesimal action of $Z\in\mathfrak k$ on $x\in X$. The moment map definition $i\_Z\omega\_X=-d\langle\mu(\cdot),Z\rangle$ says
$$
\omega\_X(Z(x),\delta x)=-\langle D\mu(x)(\delta x),Z\rangle
$$
for all tangent vectors $\delta x\in T\_xX$. By transitivity, any such writes $Z'(x)$ for some $Z'\in\mathfrak k$, and equivariance then gives $D\mu(x)(Z'(x))=Z'(\mu(x))=\langle\mu(x),[\cdot,Z']\rangle$. So the above equality becomes
$$
\begin{align}
\omega\_X(Z(x),Z'(x))&=\langle\mu(x),[Z',Z]\rangle\\
&=\omega\_{\mathcal O}(Z(\mu(x)), Z'(\mu(x))),
\end{align}
$$
where the last equality is the definition of the Kirillov-Kostant-Souriau 2-form $\omega\_{\mathcal O}$. 


