How to build an S function from a shared library generated using the system target file ert_shrlib.tlc.

I'd like to build a shared library from a proprietary Simulink model then deliver both the library and a Simulink S function using the library to a third party for analysis. The proprietary model is similar to the rtwdemo_shrlib example with constant parameters, inputs and outputs. I've tried setting the code generation interface packaging to reusable function with individual arguments but don't understand how to hand the first argument of the step function which is related to the constants. When generating the S function using the Legacy Code Tool, how would the commands look for defining the OutputFcnSpec with constant, input, and output arguments?