Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Text
ANNOTATIONTAG_TEXT
Parameter: URL
ANNOTATIONTAG_URL
Parameter: URL Info
ANNOTATIONTAG_URL_INFO
Parameter: Visit URL
ANNOTATIONTAG_URL_GO
Parameter: Color
ANNOTATIONTAG_COLOR
Parameter: Inherit from Layer
ANNOTATIONTAG_COLORFROMLAYER
Parameter: Show in Viewport
ANNOTATIONTAG_VIEWPORT_SHOW
Parameter: All Views
ANNOTATIONTAG_VIEWPORT_ALL
Parameter: Scale
ANNOTATIONTAG_SCALEFLAG
Parameter: Fold
ANNOTATIONTAG_FOLD
Parameter: Export
ANNOTATIONTAG_EXPORT
Parameter: Show all
ANNOTATIONTAG_SHOW_ALL
Parameter: Hide all
ANNOTATIONTAG_HIDE_ALL
Parameter: Hide all but this
ANNOTATIONTAG_HIDE_OTHERS