MATLAB MPC toolbox setting constraints on unmeasured outputs

I am trying to control a fourth order plant using MPC with constraints on my input, my measured output and the unmeasured states. The problem I encounter is that I cannot define the three unmeasured 'outputs' (the three other states) I have.

plantss.OutputGroup.UO = 3; % also tried plantss.OutputGroup.UV = 3;

returns an error. I also wonder how I would have to specify to which state a specific constraint belongs.

My code:

ref = 1;
Ts = 1;
plant = tf([0.5],[200 310 143 16.4 0.5]);
plantss = ss(plant);
plantss.InputGroup.MV = 1;
plantss.OutputGroup.MO = 1;
plantss.OutputGroup.UO = 3;
MV = struct('Min',-10,'Max',10);
MO = struct('Max',1.05*ref);
p = 20;
m = 3;
MPCobj = mpc(plantss,Ts,p,m,[],MV,MO);
sim(MPCobj,200,ref);