How do you generate C code from s-function using Simulink Coder?

I have a simulink model with a s-function which points to a dll. I understand that you can generate a C/C++ code from a simulink model using a Simulink Coder, but how do you do this if the .mdl contains a s-function that points to a dll? The s-function is a CarSim vehicle model by the way.