Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Text
ID_PAINTSPLINETEXT_TEXTParameter: Font
ID_PAINTSPLINETEXT_FONTParameter: Align
ID_PAINTSPLINETEXT_ALIGNParameter: Size
ID_PAINTSPLINETEXT_SIZEParameter: Horizontal Spacing
ID_PAINTSPLINETEXT_SPACE_HParameter: Vertical Spacing
ID_PAINTSPLINETEXT_SPACE_VParameter: Separate Letters
ID_PAINTSPLINETEXT_SEPARATE