Unfortunately the dde functions do not support code generation. I've made an internal note of your request so we can consider this for the future. One option is to declare dde23 as an extrinsic function using coder.extrinsic. That will allow you to use it in MEX by having the MEX call back into MATLAB to run it. If the desire is to accelerate your code and dde23 is a hot spot, then the coder.extrinsic strategy likely won't help.
function y = example(x)
coder.extrinsic('dde23');
y = preAssignOutput;
y = dde23(x,..);
end
