here is what have that works:
# Details of the runbook we are going to run
$rbid = "0E3D0830-5BB6-4717-BB94-330E0338F153"
$OrchURI = 'http://<SERVERFQDN>:81' #Replace 81 by the WebService port
# To Start a job
$body = @{
RunbookId = $rbid
CreatedBy = $null
} | ConvertTo-Json
Invoke-RestMethod -Uri ('{0}/api/Jobs' -f $OrchURI) -Body $body -Method Post -ContentType 'application/json' -UseDefaultCredentials
Currently when I try to execute a runbook via Invoke-RestMethod I receive the Error 400 Bad Request and it states "Invalid Job creation params" Can someone explain what the mandatory params for Job creation are and how the JSON body structure should look?