Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Color
HAIR_SHADER_COLOR
Parameter: Diffuse
HAIR_SHADER_DIFFUSE_STRENGTH
Parameter: Texture
HAIR_SHADER_DIFFUSE_SHADER
Parameter: Specular
HAIR_SHADER_SPECULAR_STRENGTH
Parameter: Sharpness
HAIR_SHADER_SPECULAR_POWER
Parameter: Texture
HAIR_SHADER_SPECULAR_SHADER
Parameter: Strength
HAIR_SHADER_ISPECULAR_STRENGTH
Parameter: Sharpness
HAIR_SHADER_ISPECULAR_POWER
Parameter: Texture
HAIR_SHADER_ISPECULAR_SHADER
Parameter: Roughness
HAIR_SHADER_ROUGHNESS
Parameter: Variation
HAIR_SHADER_ROUGHNESS_VAR
Parameter: Reflectiveness
HAIR_SHADER_REFLECT
Parameter: Translucence
HAIR_SHADER_TRANSMIT
Parameter: Shadow
HAIR_SHADER_SHADOW
Parameter: Alpha Depth
HAIR_SHADER_ALPHA_DEPTH
Parameter: Shadow Transparency
HAIR_SHADER_SHADOW_TRANSPARENCY
Parameter: From Hair Materials
HAIR_SHADER_FROM_MATERIAL