Navigate to your Desktop Dashboard
Click "App Settings" on the left-hand side
Click "App content"
Find the page that you wish to edit in your app, then click the pencil icon to view the buttons for that page:
Click the plus icon in the top right to add a new button
Select the type of button that you wish to add. In this example, we will add an "In-app link"
Type or paste the destination URL that you wish to set for this newly created button