I have a developer who has created a custom web application quite some time ago. They are able to give me the clientid that was registered in ArcGIS Online that the application uses to log in.
I want to find this item in ArcGIS Online to register it in the AppLauncher.
How can I find out the item id from just the clientid? Searching by the client id doesn't return anything, and I can't seem to find the item by trawling through content items.
You can use the Registered App endpoint https://developers.arcgis.com/rest/users-groups-and-items/registered-app.htm to perform this kind of reverse lookup.
The token is required and only the item owner or an organization administrator can perform this action.
Shameless plug... my company provides a free utility called Geocortex Item Manager that provides a friendly user interface for this.