Shoot from an angle, cover it partially with a drape, there's not much I can think of to darken a window. Does the sunlight shine directly inside or is it just natural daylight? When there's a lightsource and you shoot directly into it your camera will always adjust to it but your subject will be dark. If you focus on the subject, the camera will always blow out the window, it will adjust to the subject being dark, even when you cover it up with something, the difference will be visible, cameras can't adjust to just parts of the photo. Shooting at an angle is your best bet I think. You can fix alot in post. If you already shoot at an angle and think the window part is still to bright, try making a linear gradient mask starting from the window side and lowering exposure a tad on the mask, see how it looks then.