Hey Mike, thanks for making use of the Forum!
From a physics point of view, why would you want to directly specify the pressure drop across the filter? I don’t think I have seen such approach in the past (perhaps someone with more CFD experience can correct me).
If you just want to specify the pressure after the filter, you can cut your model there and use a Pressure Inlet condition, but then you would leave out the air box model.
Rather than this, the approach I would suggest is to use a Porous Media concept, which will locally compute the pressure drop as a function of the flow speed, just like a filter would do. This way you can have a more precise solution, and perhaps implement the filter pressure drop curve if you have it (otherwise you can approximate the pressure drop model).
Please refer to the following pages for more details on how to implement this approach: