$username = "myemail@yahoo.it"
$password = "myaccessKey"
$url =  "https://prsrl.od2.vtiger.com/webservice.php?operation=getchallenge&username=" + $username
$challenge = Invoke-RestMethod $url
$accessKey = $challenge.result.token + $password
$md5 = new-object -TypeName System.Security.Cryptography.MD5CryptoServiceProvider
$ascii = new-object -TypeName System.Text.ASCIIEncoding
$hash = [System.BitConverter]::ToString($md5.ComputeHash($ascii.GetBytes($accessKey)))
$hash = $hash -replace '-', ''
$body = @{operation='login'
    username=$username
    accessKey=$hash}
$url = "https://prsrl.od2.vtiger.com/webservice.php"
$login = Invoke-RestMethod -Body $body -Method POST -ContentType 'application/x-www-form-urlencoded' $url
$login