Execute an action:

   $url= "https://<FQDN>/vco/api/actions/SopraSteria.ServiceNow/SN_ExportData/executions"
   $headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
   $headers.Add('Accept', 'application/xml')
   $headers.Add('Content-Type','application/json')
   $headers.Add('Authorization',"Bearer $vROToken")
   $respon = Invoke-WebRequest -Uri $url -Method POST -Headers $headers -Body '{}';
   Write-Host $respon

Execute an action with parameters

   $url= "https://<FQDN>/vco/api/actions/SopraSteria.ServiceNow/SN_GetReservations/executions"
   $headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
   $headers.Add('Accept', 'application/xml')
   $headers.Add('Content-Type','application/xml')
   $headers.Add('Authorization',"Bearer $vROToken")

   $body = '<execution-context xmlns="http://www.vmware.com/vco">
    <parameters>
        <parameter type="string" name="CustomerID" scope="local"><string>34</string></parameter>
        <parameter type="string" name="EnvironmentID" scope="local"><string>D2</string></parameter>
        <parameter type="string" name="ServerType" scope="local"><string>WEB</string></parameter>
    </parameters>
</execution-context>';

   $respon2 = "";
   $respon2 = Invoke-WebRequest -Uri $url -Method POST -Headers $headers -Body $body;
   Write-Host $respon2

Execute an workflow:

   $url= "https://<FQDN>/vco/api/workflows/484702ca-94ee-4838-87b0-614ed374ff2a/executions"
   $headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
   $headers.Add('Accept', 'application/xml')
   $headers.Add('Content-Type','application/json')
   $headers.Add('Authorization',"Bearer $vROToken")
   $respon = $null
   $respon = Invoke-WebRequest -Uri $url -Method POST -Headers $headers -Body '{}';
   Write-Host $respon

NOTICE: Actions can be called by name, Workflows must be called by the ID!