Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Text
ANNOTATIONTAG_TEXTParameter: URL
ANNOTATIONTAG_URLParameter: URL Info
ANNOTATIONTAG_URL_INFOParameter: Visit URL
ANNOTATIONTAG_URL_GOParameter: Color
ANNOTATIONTAG_COLORParameter: Inherit from Layer
ANNOTATIONTAG_COLORFROMLAYERParameter: Show in Viewport
ANNOTATIONTAG_VIEWPORT_SHOWParameter: All Views
ANNOTATIONTAG_VIEWPORT_ALLParameter: Scale
ANNOTATIONTAG_SCALEFLAGParameter: Fold
ANNOTATIONTAG_FOLDParameter: Export
ANNOTATIONTAG_EXPORTParameter: Show all
ANNOTATIONTAG_SHOW_ALLParameter: Hide all
ANNOTATIONTAG_HIDE_ALLParameter: Hide all but this
ANNOTATIONTAG_HIDE_OTHERS