unable to authenticate using javascript/jQuery

[PrestoDigital]

Using the sandbox APIendpoint provided (any my name/key which are obfuscated here), I get a generic error return message

function testAuthorize(xx)
{
$.post(APIendpoint,
{
contentType: 'application/json',
data: JSON.stringify({"authenticateTestRequest":{"merchantAuthentication": {"name": "xx","transactionKey": "yy"}}})
})
.done(function(wtf)
{
rxjson=wtf;
debugger;
});
}