Most clients have a "View this email online link" as standard in their templates. Please contact your account manager if you would like this to be available in every template.
If you would like to manually add this link to your mail. Create the text using the text editor:

Highlight the text you want your clients to click on and press the link button.

Type # where it requests a link and press insert.

When you send the email, this link will now automatically open the email in a web-browser.