Well, there's no easy way to do it, so basically just live with it and don't send those spaces. mIRC code will still work and those leading spaces will be added automatically again once you check the code (the {} button in the editor) and save it. It's a limitation in mIRC, one that probably won't ever be overcome since it requires developing a new scripting language...
Some people will advise you to replace spaces with another character, $chr(160), because some fonts have nothing in that position. Other fonts have strange characters there, like a capital with accents on it. And most importantly, it will break any code you send.
Another way is to include 2 ctrl-b characters (bold) between each pair of spaces, as this will show correctly on screen at a minor cost of extra message length and it not working on channels that don't allow color codes. Still, any code sent using such a script will be broken since ctrl-b ctrl-b is not a valid command.
