Constrain the norm of output variables in MPC controller

I have a state vector which is 3-dimensional vector. In the mathematical model, this vector should remain a unit vector. However, during the update of the state equation and integrating it, there is a chance that may no longer remain a unit vector.

How can I use the object to set this constraint?

nlobj.OutputVariables(3)._____ = ?          %x component of e_i
nlobj.OutputVariables(4)._____ = ?          %y component of e_i
nlobj.OutputVariables(5)._____ = ?          %z component of e_i