I believe it's the last parameter:
void ActionStartConversation(object oObjectToConverse, string sDialogResRef = "", int bPrivateConversation = FALSE, int nConversationType = CONVERSATION_TYPE_CINEMATIC, int bIgnoreStartRange = FALSE, string sNameObjectToIgnore1 = "", string sNameObjectToIgnore2 = "", string sNameObjectToIgnore3 = "", string sNameObjectToIgnore4 = "", string sNameObjectToIgnore5 = "", string sNameObjectToIgnore6 = "", int bUseLeader = FALSE);
I'm guessing it's executed through a custom OnDialogue script for the NPC, but I'm not sure what the exact code would be.