While not directly promoting DeX, the Galaxy Apps store does promote your DeX-compatible Android apps via:
Galaxy Apps in-store promotions
For details, see Promoting within Galaxy Apps.
Promotions outside Galaxy Apps
For details, see Promoting outside Galaxy Apps.
These promotions are based on information, graphics, and videos registered with your apps. Samsung also assists you in promoting your apps via advertising channels outside the Galaxy Apps store (including download discounts and coupon codes).
You can promote your app's DeX compatibility in your app and media channels.