The ability to add fields in ArcGIS Online hosted feature services is useful when there is a need to include additional data to published data.
Note: The ability to add fields is only available for hosted feature layers. Feature layers that are not hosted do not have the Add Field option in the attribute table, and have no Data tab to add or alter data.
To add a field to an ArcGIS Online hosted feature service, use one of the methods described.
Note: Only the owner of a hosted feature layer, or an administrator of the organization is able to make changes to the attribute schema.
From the attribute table in ArcGIS Online Map Viewer Classic
Note: To add or edit values in the newly created field, navigate to the hosted feature layer's item details page, and click the Data tab > Field. Double-click the desired field attributes and make the necessary edits.
From the Table view of the Data tab
From the Fields view of the Data tab