Input arguments to Simulink executable

How do I define input arguments to a standalone executable generated in Simulink? For example, I would like to use an input argument to specify the final simulation time, say

./mymodel 100

where mymodel is the executable generated from Simulink and 100 would be the final simulation time. I tried to place an input port on the top-level of my model. The model compiles, but it doesn't seem to respond to the input. That leads me to my second question: the value 100 above is as far as I know passed as a string. How would I convert a string to a numerical value in Simulink?