You can add a button to your email campaigns that takes the recipient to any of the web pages you've created for your event in the Website Builder, and these buttons are called "Event page link buttons".
To set this up, open up your email campaign and navigate to the email builder. Find the Event page 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 text inside the button to select it, and click the cogwheel icon found on top of the right hand panel to reveal component settings.
Under component settings, select the page you'd like this button to lead to in the Page dropdown field. Note that the selection of pages here are those published via the Website module.
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.