Hi @ablomfield,
Thanks for your patience so far!
The above API endpoint provided by @dstaudt is actually correct, but I believe when you're trying to list out the organizations with the API where you're already "External Admin", exactly that time you're not logged into that Org account in Control Hub.
Please note, this "External Admin" role is not permanent one to a User for an Org, once you login to the Control Hub of that Org - you get the role assigned as "External Admin", but that's only for a temporary basis and the external administrators are nothing but the Read-only administrator of your Org.
Once your login session destroys for that Control Hub Org account, your "External Admin" role to that Org is also gone.
You can find the same has been documented here as well - https://help.webex.com/en-us/article/fs78p5/Assign-organization-account-roles-in-Control-Hub#id_87926
I've highlighted the specific areas in the attached screen shot too which should help here.
So, you can login to those Orgs for the temporary basis and this /list-organizations API as well as /get-organization-details API would help you find the details of those Orgs.
Let us know for any further assistance on this.
Regards!
Sandip
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