Easiest thing is just put the URL of the link in the body of the email. It is just text after all.
If you want it shown as a “clickable” link, you have to code it using HTM – but then you need to have (or should have) the proper headers in the email to indicate the email is in an HTML format. Specifically the “MIME-version” and “Content-type” likely need to be specified.
Lots of references on the web for email header formats – Google is your friend.