Here's a summary of my interactions with esri support:
# if you are using a hosted layer:
You can use addToDefinition to update the layer you want with the required index. This will then magically work as expected
If you are using non-hosted (arcgis server) services
1. you can put a % in front of your searches. (haha - tell your non techy staff to remember that one)
2. you can configure a custom app to add a prefix % to the search widget - this would not apply to expb but would be a custom javascript app you'd write
3. you can also configure a custom app use an interceptor to search for all requests that contain the LIKE clause and apply the leading percent character.
See some notes here:
https://community.esri.com/t5/arcgis-online-questions/searching-adresses-requires-the-whole-exact-ph...