When you click on the empty box, you’ll be presented with the form shown on the right. In order to add a design to the current Issue, you’ll need to:
Copy the link from a Figma file, paste it on the File Link box, and click
SendYou’ll be shown a list of Pages, Frames, Groups, on that file. Select the one you want to render.
The Preview box will show you the rendered element. If you are happy with your selection, click
Save
You’ll be switched to the main view with the list of all the designs added to this Issue.