Karsten, you can also do that in the webmap. Open the webmap in the viewer Start editing Open the feature-management (manage new features) If necessary add new types At the featuretypes you don't want to see when creating new features remove the templates It looks like that Thats the result in the map viewer in the WAB edit widget After creating, you can select all types in the popup If you use ArcGIS Pro you can delete the Symbols from the feature template before sharing the map or layer.
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