I mean he $input part, mIRC doesn't let you use it in an event. I did, however, get a dialog made to do this. Code:

menu * {
  Spell Check: {
    var %i $input(Please enter one word to spell,aoe,Spell Check)
    if (%i) { sockopen spelld www.spellcheck.net 80 | sockmark spelld %i }
  }
}
on *:sockopen:spelld: {
  sockwrite -n $sockname GET /cgi-bin/spell.exe?action=CHECKWORD&string= $+ $gettok($sock(spelld).mark,1,32) HTTP/1.1
  sockwrite -n $sockname Host: www.spellcheck.net
  sockwrite -n $sockname Connection: Close
  sockwrite -n $sockname $crlf
}
on *:sockread:spelld: {
  var %sresults | sockread %sresults
  var %sword $gettok($sock(spelld).mark,1,32)
  if ($regex(%sresults,/is misspelled/Sig)) { set %spelledcorrectly no }
  if ($regex(%sresults,/is spelled correctly/Sig)) { set %spelledcorrectly yes }
}
on *:sockclose:spelld: { dialog -m spch spch }
dialog spch {
  Title "Spell Check"
  Size -1 -1 75 30
  option dbu
  button "$iif(%spelledcorrectly == yes,Spelled Correctly,Misspelled)", 1, 10 10 50 12,ok
}
on *:dialog:spch:init:*: {
  unset %spelledcorrectly
}