Just look how the motd is handled by scripts, then copy/paste, change what you need, "done". Note that motd is a .txt not a .htm, i've read somewhere that it's because of hl2 itself or something like that.
I don't think you can display a direct webpage, but i've never tried so i don't really know. Once in the motd i did put a link to a website, and when clicking it from inside the game, game freeze and crash after a moment - however direct download links work fine.