Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Color 1
STARSHADER_COLOR1
Parameter: Color 2
STARSHADER_COLOR2
Parameter: Streaks
STARSHADER_STREAKS
Parameter: Inner Radius
STARSHADER_INNERRADIUS
Parameter: Outer Radius
STARSHADER_OUTERRADIUS
Parameter: Density
STARSHADER_DENSITY