markco -

what you're asking sounds to me like you would simply need to add the alias for mydomain.com to the 123example.com site. You can do this by going to site settings while logged in, then scrolling down to the last menu entry "portal aliases". simply add the domain to this list, and the site will reflect it when the user acceses the site from that alias.

If you're asking about the formatting of the alias, you can also change that in the language packs under "host > languages > language packs" i believe.

Portal aliases simply work to serve content to multiple URL's.  If you setup dotnetnuke under http://www.mysite.com/dotnetnuke, when you point http://www.yoursite.com to the portal, you must access the site via http://www.yoursite.com/dotnetnuke.  If you are running multi portal I strongly recommend using a ROOT installation of dotnetnuke.