You can add a button to your email campaigns that takes the recipient to any external site you'd like, and these buttons are called "External site link buttons".
To set this up, open up your email campaign and navigate to the email builder. Find the External site link button block under the Buttons section in the right hand panel, then drag and drop it where you'd like the button to be in your email template.
Now click once on the button and select it (note that you need to select the button itself, not the whole section), and click the cogwheel icon found on top of the right hand panel to reveal component settings.
Under component settings, type or paste the URL you'd like this button to lead to in the Href field. You can also set whether you'd like this link to be opened in the active window/tab when clicked or a new one by changing the Target setting.
You can also update the text on the button to anything you'd like by double clicking on the text and typing your own.
Remember to publish your email template when you're done.