I wish I could tell you this is easily done, but it isn't. HTML email is a whole different beast from an HTML website, as I had the pleasure of learning earlier this year. Please read up on this link before you actually write an HTML email.
This next link shows compatibility lists of email clients and what version of CSS they can handle.
This last link is so you can test your email before you send it out to see if there are any errors, or to get it to clean up your html and optimize it for email.
Now that you see html email is much more of a hassle, you may realize that it won't be possible to use code that executes client side - it's just not safe and no one really allows it in email. If you are using php, then this is your easy solution. Decide on a good variable name, and define it. Then place that variable each time you want the link. PHP will work because it is executed server-side, so you won't have to worry about any PHP code reaching the client. Here is a super easy example that will output "Here is a link to likeplum: http://www.likeplum.com."
$link = "http://www.likeplum.com";
echo "Here is a link to likeplum: $link";