  We show that supersymmetry can be used to compute the BCFT one-point function
coefficients for chiral primary operators, in 4d $\mathcal{N}=2$ SCFTs with
$\frac{1}{2}$-BPS boundary conditions. The main ingredient is the hemisphere
partition function, with the boundary condition on the equatorial $S^3$. A
supersymmetric Ward identity relates derivatives with respect to the chiral
coupling constants to the insertion of the primaries at the pole of the
hemisphere. Exact results for the one-point functions can be then obtained in
terms of the localization matrix model. We discuss in detail the example of the
super Maxwell theory in the bulk, interacting with 3d $\mathcal{N}=2$ SCFTs on
the boundary. In particular we derive the action of the SL(2,$\mathbb{Z}$)
duality on the one-point functions.
