According to https://en.wikichip.org/wiki/mirc/commands/server this should work:

Code
on *:start: {
  server efnet.port80.se:6667 -j #chan,#chan2 key1,key2
}


Note: Use * to specify no key is needed for that channel.

GNU Terry Pratchett - Looking for a mIRC help channel -> Check #mircscripting @ irc.swiftirc.net