ArcGIS Online does not prompt for credentials when accessing the settings page of a web map containing a secure service, which causes the Enable Search option to be missing.

Workaround

1. Change the relevant service on ArcGIS Server, which is being consumed by ArcGIS Online, to non-secure (public).
2. Go to the settings of the web map in ArcGIS Online that consumes this service. Configure the searchable layers and define which fields should be used for search.
3. Save the web map.
4. Switch the relevant service on ArcGIS Server back to secure (private).

Steps to Reproduce