Code:
Ext.MessageBox.show({
         title: 'my title',
         icon: Ext.Msg.QUESTION,
         msg: 'text here</br> other text in new line?',
         buttonText: { yes: "Yes", no: "No" },
         fn: function (btn) {
           if (btn == 'yes') {
            //your realization
           }
         }
       }
     });