Use beforeSend
:
$.ajax({
url: "http://localhost/PlatformPortal/Buyers/Account/SignIn",
data: { signature: authHeader },
type: "GET",
beforeSend: function(xhr){xhr.setRequestHeader('X-Test-Header', 'test-value');},
success: function() { alert('Success!' + authHeader); }
});
http://api.jquery.com/jQuery.ajax/
http://www.w3.org/TR/XMLHttpRequest/#the-setrequestheader-method