So this is sort of a weird function, and it confuses a lot of people. I'm changing the way it works in Chipmunk 7 to help fix that.
Here's how it works. It's an offset from the center of gravity in world coordinates. So the impulse is applied to the body at (body.position + offset) in world coordinates. The impulse itself is also in world coordinates.
Also keep in mind that the shape's vertex coordinates are specified in body local coordinates where (0, 0) is the center of gravity of the body. Passing window coordinates into that is probably not what you want. You probably want to set the body's position to the center of the window and then make your shape's vertexes centered around (0, 0).