Accessing website with certificate authentication from Powershell

I am using powershell script which sends xml message to a web service and to send data it needs to provide certificate for authentication.  I have got this to work if I just have one cert under cert:\LocalUser\My  location but let's say I have multiple certs, how do I choose the correct one to use.  Here is the code I'm using to send XML message

    $parameters = "<message>......</message>"
    $http_request = New-Object -ComObject Msxml2.ServerXMLHTTP.6.0
    $http_request.setOption(2, 13056)
    $http_request.open('POST', $URL, $false)
    $http_request.setRequestHeader("Content-type", "text/xml")
    $http_request.setRequestHeader("Content-length", $parameters.length)
    $http_request.setRequestHeader("Connection", "close")
    $http_request.send($parameters)

The above works because if I have one cert, but it doesn't work if I have multiple cert under cert:\LocalUser\My  location because it has to use the correct cert which is not the case by default.  Is there a way to do this? 

Also another optional question, you can discard if you don't know this answer: How is the best way to sign this script with the same cert or covert this cert to make it code signing cert?

Thanks!!!

sjoshi