Values entered in the Model Initialization Commands or Mask Initialization Commands can be passed into C-Scripts using the "Parameters" field, configured in the C-Script's "Setup" tab.
From the C-Script documentation, the "Parameters" field is described as: "A comma-separated list of expressions that are passed as external parameters into the C functions. The expressions can reference workspace variables and must evaluate to scalars, vectors, matrices, 3d-arrays or strings."
You can then access the parameter entries using the ParamRealData() or ParamStringData() macros.