How to Pass UserName & Password in Ajax

var _url = "http://localhost:49497/DataService.svc/Mst_User?$filter=Us_Name eq 'Basit' and Us_Password eq 's'&$format=json";

  $.ajax({
                  type: "POST",
                  data: "{ strFUserName: '" + VariableUsername + "', strFPassword: '" + Variablepassword + "'}",
                  url: _url,
                  timeout: 10000,
                  async: true,
                  dataType: "json",
                  contentType: "application/json; charset=utf-8",
                  success: function (data)
                      debugger;
                      //
                      if (data.d.length > 0 && data.d[0].Us_Admin == 1) {

                          var Adminrole = data.d[0].Us_AdminModule
                          var UsNo = data.d[0].Us_No


                      } else {
                          Alter(‘Login Failed’)
                      }
                  },
                  error: function (xhr, status, error) { // flow always comes in error callback even url returns true and this is the issue.
                          Alter(‘Login Failed’)
                  }
              });
          }
      };