Depends on where you are.
ZPostPeerChat = battle
ZPostChannelChat = channel
ZPostStageChat = stage
You'll need to check where the player(s) are at before, such as;
if( pMyChar->nPlace == MMPL_LOBBY )
ZPostChannelChat( pMyChar->GetUID(), szMsgBuff );
else if( pMyChar->nPlace == MMPL_STAGE...