You can remove the "licensed for developer us only" watermark by setting a "Lite" level license for you app. To do this simply follow these steps.
In Login to AppStudio desktop and upload your app your ArcGIS organization
Next, in AppStudio Desktop browse to Settings > Licensing for your app, and click the Set Lite license button. Sign in to ArcGIS Online if you haven't already, and the License key field will automatically be populated. Then, click the Apply button in Settings to save the changes. After you hit apply upload your app again to your ArcGIS organization.
you can find more information about licensing here: License your app—AppStudio for ArcGIS | Documentation
If you have other AppStudio questions I suggest posting them on the AppStudio GeoNet page: https://community.esri.com/groups/appstudio