To balance a studio flash (softbox mount in this case) to the ambient or outdoor light is actually quite simple. Just take a meter reading with your camera before taking the shot. If you take a shot using aperture priority mode the inside and your model will be quite dark but the ambient light, or outdoors, will be correctly exposed. In this case my camera told me that I should use aperture 22 (or thereabout) if I use a shutter speed of 1/160 s which is my standard setting for flash photography.
Then it is just a matter of setting your flash to f22.
Well, after that you just get to try to get the model to sit without making faces while you make the flash not to make terrible reflections in the window. I did so, so… It would be much easier if you place your model not directly in the window frame or/and use your polarizing filter.