Add two boundry at the same face

Open the thermodynamic properties option closer to the bottom of the Flow Rate BC definition.  This pressure is only an approximate pressure, defaults to your ambient/environmental setting, because you cannot set a pressure and flow rate at the same time.

Set another boundary with a pressure and this will help set the state of the fluid, and with it you can find the pressure differential to the flow rate by setting a  Surface Goal of pressure on the flow rate face.  Use an Equation Goal to subtract the calculated pressure at the flow rate face from the ambient pressure (hint: select the Input Data folder on the tree and then it will present you a list of conditions set for the environmental conditions in the Wizard).