How To: Add coded value domains to a hosted feature service in ArcGIS Online
The instructions provided describe how to add coded value domains to a hosted feature service in the REST endpoint in ArcGIS Online. The instructions also can be applied in other instances, such as editing the values inside of a coded value domain for the feature service.
Users may have to visit each layer to update the parameter if there are multiple layers in the service.
Ensure the feature service is not shared publicly before using this workflow.