Authentication error from API using ajax query

Does anyone know how the JSS API expects credentials from an ajax query? This is using a read-only account to gather machine information through a SharePoint page. The below code is in-place but every iteration returns an authentication error. (This doesn't include the text field where a user would enter a hostname). I know that the account is correct because I can successfully GET with curl, but not through ajax.

$.ajax({
                    type: "POST",
                    url: apiURL,
                    async: false,
                    dataType: "json",
                    contentType: "application/json; charset=utf-8",
                    processData: false,
               headers: {
                                "Authorization": "Basic " + btoa("username:password")
                                },
                    success: function (){
                                alert('Thanks!');
                    }
                });