Decline buttons are used to enable your participants to decline invitations, or cancel their registration if they've already registered.
To add a decline link button to your email template, open up your email campaign and navigate to the email builder. Find the Decline 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.
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.