Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Text
ID_PAINTSPLINETEXT_TEXT
Parameter: Font
ID_PAINTSPLINETEXT_FONT
Parameter: Align
ID_PAINTSPLINETEXT_ALIGN
Parameter: Size
ID_PAINTSPLINETEXT_SIZE
Parameter: Horizontal Spacing
ID_PAINTSPLINETEXT_SPACE_H
Parameter: Vertical Spacing
ID_PAINTSPLINETEXT_SPACE_V
Parameter: Separate Letters
ID_PAINTSPLINETEXT_SEPARATE