I guess that you are referring to putting a url in the body of an email although that isn't really clear.
If that is what you mean then it isn't under your control, many email programs have their own user interface that doesn't use the web, you simply can't make a web page open in something like TAO or Pegasus because they aren't designed to work like that. The former does not understand html at all and the latter will open a window in your default browser.
Webmail programs like Yahoo are designed to keep the session open when the user clicks on a link so always force a new window or tab to open. That is forced by the provider and will override anything you try to code in the email.
