API Requests failing when using sessionKey authentication

I'm trying to follow the documentation Web Help Desk REST API Guide  to authenticate with a sessionKey using PowerShell.  I've tried the same steps in PowerShell, CURL, and the Postman API test bench - all return the same failure.

I'm able to successfully acquire the sessionKey by issuing a request like this:

Invoke-WebRequest -Uri "https://<WHD URL>/helpdesk/WebObjects/Helpdesk.woa/ra/Session?username=WHDAdmin&apiKey=<hidden>"

The response is valid JSON, and contains what appears to be a valid session key:

{"type":"Session","sessionKey":"<sessionKey>","currentTechId":1,"instanceId":-1}

Then I issue the request for a ticket:

Invoke-WebRequest -Uri "https://<WHDURL>/helpdesk/WebObjects/Helpdesk.woa/ra/Tickets/100?sesionKey=<sessionKey from last step>"

The response is not a JSON object of the requested ticket as expected, but rather an error:

Invoke-WebRequest : Authentication required. At line:1 char:1 + Invoke-WebRequest -Uri "https://<WHDURL>/helpdesk/WebO ... + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~     + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-WebRequest], WebException     + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand

Has anyone successfully used the sessionKey authentication method against a WHD instance?

Any help would be appreciated.

Thanks,