how to pass credentials for a webservice using jquery ajax call?

Try using post for the method type, most webservices are secured and require transimssion using post and not Get

plus to help you debug the error and a response text to your error.

 $.ajax({
     url: WEBSERVICE_URL,
     type: "POST", //This is what you should chage
     dataType: "application/json; charset=utf-8",
     username: "admin", // Most SAP web services require credentials
     password: "admin",
     processData: false,
     contentType: "application/json",
     success: function () {
         alert("success");
     },
     error: function (xhr, ajaxOptions, thrownError) { //Add these parameters to display the required response
         alert(xhr.status);
         alert(xhr.responseText);
     },
 });

If you are doing cross domain request:

$.ajax({
    url: "yoururl",
    type: "GET",
    dataType: 'json',
    xhrFields: {
         withCredentials: true
    }
});