Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Mode
MDATA_MWS_EDGEMODEParameter: Add to Selection
MDATA_MWS_ADD_TO_SELParameter: Tag Name
MWS_TAG_LINKParameter: Auto Set Selection
MWS_TAG_AUTO_SET_SELParameter: Set Selection
MWS_TAG_SET_SELParameter: Add Selection
MWS_TAG_ADD_SELParameter: Subtract Selection
MWS_TAG_REM_SEL