Here is the answer in case anyone is looking for it.
n = 16;
k = 12;
prim_poly= [1 0 0 0 1 1 1 0 1];
gen_poly = rsgenpoly(255,251,285,0);
this.system_obj_encoder = comm.RSEncoder('MessageLength', k, ...
'CodewordLength', n, ...
'PrimitivePolynomialSource', 'Property',...
'PrimitivePolynomial',prim_poly,...
'GeneratorPolynomialSource', 'Property',...
'GeneratorPolynomial', gen_poly);
And then of course you use the step member function to decode.