For starters, you could put this into your alias tab because that's what it is. You just have to put this on a new line:
/id /ns id %pass
If you're too lazy for that, literally all you have to do is type "/alias /id /ns id %pass" into your editbox on mIRC.
Then press alt+O > connect > options > perform
add "/id" to the perform commands and it will identify you upon connecting.