The following function can be used to perform a HTTP POST in PowerShell. To use it simply, call the PostWebRequest(...) function with the appropriate parameters.

# Function to help post HTTP request to web service
Function PostWebRequest([String] $url, [String] $data, [int] $timeout)
{    
    $buffer = [System.Text.Encoding]::UTF8.GetBytes($data)
    [System.Net.HttpWebRequest] $webRequest = [System.Net.WebRequest]::Create($url)
    $webRequest.Timeout = $timeout
    $webRequest.Method = "POST"
    $webRequest.ContentType = "application/x-www-form-urlencoded"
    $webRequest.ContentLength = $buffer.Length;


    $requestStream = $webRequest.GetRequestStream()
    $requestStream.Write($buffer, 0, $buffer.Length)
    $requestStream.Flush()
    $requestStream.Close()


    [System.Net.HttpWebResponse] $webResponse = $webRequest.GetResponse()
    $streamReader = New-Object System.IO.StreamReader($webResponse.GetResponseStream())
    $result = $streamReader.ReadToEnd()
    return $result
}