"how can I relate the velocity inlet with the pressure inlet in my boundary condition?"
Whenever you define the pressure inlet boundary condition, be aware that the value you assign is the stagnation pressure and not the static pressure at the inlet. Using the equation of the stagnation pressure (depending on the type of flow), the code computes the velocity components using the static pressure from the previous iteration. So ultimately, it will behave like a velocity inlet condition but with some manipulation.
For incompressible flow: