How to use API to access Open-Audit

The Open-AudIT API says:

You can request a cookie by sending a POST to the URL below containing username and password attributes and values:

XML

<username>admin</username>

<password>password</password>

JSON

{"username": admin, "password": password}

Any ideas/suggestions on how to make the below vbscript work?

Set result = CreateObject("ADODB.Stream")
result.Type = 2
result.Open
result.Position = 0
result.Charset = "UTF-8"
result.WriteText "<?xml version=""1.0"" encoding=""UTF-8""?>" & vbcrlf
result.WriteText "<username>" & "admin" & "</username>" & vbCrLf
result.WriteText "<password>" & "password" & "</password>" & vbCrLf

url = "http://192.168.XX.XX/open-audit/index.php/login/login_auth"
Set objHTTP = WScript.CreateObject("MSXML2.ServerXMLHTTP.3.0")
objHTTP.setTimeouts 5000, 5000, 5000, 120000
objHTTP.SetOption 2, 13056
objHTTP.Open "POST", url, False
objHTTP.setRequestHeader "Content-Type","application/x-www-form-urlencoded"
result.Position = 0
objHTTP.Send "form_systemXML=" + urlEncode(result.ReadText()) + vbCrLf

objHTTP.ResponseText

Please help us to solve the problem.
Thanks for your advice.