Powershell – How to start a runbook using web service call to SC Orchestrator in Powershell

I have System Center Orchestrator 2012 R2 and I want to start a Runbook on the runbook server by calling the web service. The orchestrator console works fine if I browse to it like so: http://orchestratorserver.my.domain:82

When I first browse to it I get prompted to enter my domain credentials, only then does the Orchestrator Console page load.

If I call this PowerShell script, I get the following error and I'm wondering if it is related to the credential prompt? There are no firewalls on the orchestrator runbook server, or the host I'm calling from.

Exception calling "GetResponse" with "0" argument(s): "The remote server returned an error: (405) Method Not Allowed."