var credentials = {
    "username": "usr123",
    "password": "pass123"
};

$.ajax({
    type: "POST",
    url: "<endpoint><end-point>?action=post&module=user&resource=login",
    data: JSON.stringify(credentials),
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function(data){alert(data);},
    failure: function(errMsg) {
        alert(errMsg);
    }
});