This code can be used to scroll a string of text from one end to the other. Unfortunately, due to mIRC's space character limitations, you can't make it appear to be scrolling across like the <marquee> tag. You may be able to make the marquee work properly with the use of a dll like spaces.dll.

Code:
alias mq.start {
  ;*** Call to start the scrolling
  unset %mq.*
  set %mq.point 0
  .timer.mq -m 0 500 mq.disp
}

alias mq.stop {
  unset %mq.*
  .timer.mq off
}

alias mq.disp {
  ;*** Scrolls one character for each call (chr(32) ignored)
  var %mq.title = This is the text to scroll in the textbox
  inc %mq.point 1
  if (%mq.point > $len(%mq.title)) set %mq.point 1
  if ($mid(%mq.title,%mq.point,1) == $chr(32)) inc %mq.point 1
  var %mq.disp = $right(%mq.title,$calc($len(%mq.title) - %mq.point)) $left(%mq.title,%mq.point)
  echo -a did -ra <id_of_dialog_item> %mq.disp
}


Change the value of %mq.title to scroll whatever you want.

/sb.start to start the scroll action.
/sb.stop to stop it.

-genius_at_work