For Windows users looking for a PowerShell alternative, here it is (using POST). I've split it up onto multiple lines for readability.
$url = 'https://sandbox.mediamind.com/Eyeblaster.MediaMind.API/V2/AuthenticationService.svc'
$headers = @{
'Content-Type' = 'text/xml';
'SOAPAction' = 'http://api.eyeblaster.com/IAuthenticationService/ClientLogin'
}
$envelope = @'
<Envelope xmlns="http://schemas.xmlsoap.org/soap/envelope/">
<Body>
<yourEnvelopeContentsHere/>
</Body>
</Envelope>
'@ # <--- This line must not be indented
Invoke-WebRequest -Uri $url -Headers $headers -Method POST -Body $envelope