Prescribe a value for a fixed point

Is there a method to do this in FreeFEM++?

For example, fix the velocity at the centre of a disc so that it can keep rotating stably, or give a pressure reference point for a flow problem.

Some one suggested do u[][n]=the value. I have tried this, and found that u[][n] was erased after solving the FEM system.

Thank you for any thought in advance!