Additional dependent variable laminar flow

Hello Comsol Community,

I try to simulate a laminar airflow in a rectangular channel. The air enters the domain at the bottom of the channel and leaves it at a lateral wall. In this simulation I want the vertical force (integrated pressure) on the top of the channel to equal a specific value. To fulfil this constraint the normal velocity at the inlet should
be a variable and solved during computation. How can I add the inlet velocity as an dependent variable and the extra equation that completes the system of equations. Is there a possibility to add dependent variables to predefined physics. Do you have any advices for me?

Thank you very much in advance.
Best regards
Maximilian