can i use the 'Add to Definition' REST api to update a domain on a feature service, i want to add in an additional value into the domain
You can also add coded value domains through a list of values from the data tab in ArcGIS Online. Check out these resources for more information:
Define attribute lists and ranges—ArcGIS Online Help | ArcGIS
You will have to use the Update definition operation. I have included a link to a very helpful document outlining what can be updated via this definition, including domain values.
How To: Add coded value domains to a hosted feature service from a REST endpoint
Hope this helps!
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