See the link below for what you need to do to license your app and remove the dev watermark.
License your app—ArcGIS Runtime SDK for Qt | ArcGIS for Developers
With a developer account, you can download and install an ArcGIS Runtime SDK and immediately have access to all of the functionality in the API for development and testing purposes. However, any map in your app will be watermarked (as pictured below) and debug messages will appear saying that your app is licensed for developer use only.