Smart watches that can use an image as a watch face typically have an interface for allowing you to add your own image from a variety of sources. Mine has an app that I can access on my phone that allows me to add an image from Photos (I have an Android phone) or select Creative Cloud as a different app. When I select Photos, I can use any image I save there--including images I make from Adobe Express.
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I should add that if you're creating an image using Firefly, you can download it by clicking on the Share icon and selecting Download from the More Options dropdown menu so that you can save it somewhere that the app for your smart watch would allow you to access it.