It's "mode # +v $nick" - and a good way to flood yourself off in case of many join events, e.g. after a netsplit
mIRC's autovoice feature will reduce this risk (in address boock > tab "control" > "voice" > add the #channel and a wildmask like *!*@*, and check "random delay op/voice")
However, network services may do it for you if your network features "ChanServ":
Try "/msg chanserv help levels". If you get a reply, have a look at "/msg chanserv help levels desc". If you find "autovoice" in the list of levels, set it to "0", via "/msg chanserv levels #yourchannel set autovoice 0".