This article explains how to insert a link in rich text or a URL in plain text emails with Mac OS X Mail. Information on editing or removing a link is included.
Inserting a link to a web page is easy in Mail for macOS. Just copy the web page URL from your browser’s address bar and paste it into the body of the email. However, the way Mail formats outgoing messages can interfere with the URL you’re trying to send. One workaround is to create a hyperlink.
To add a clickable link, you must compose your message as rich text.
Open Mail and start a new message.