设置TextArea的emptyText属性，如下所示：

var myMsg = Ext.Msg.show({
                title: 'Approve Confirmation',
                message: 'Confirm to approve the selected items?',
                multiline: 'true',
                prompt:{
                   placeHolder:"PLACEHOLDER TEXT HERE",
                },
                iconCls: 'fa fa-check-square-o',
                buttonText: {
                    ok: 'Yes',
                    cancel: 'No'
                },
                height:350,
                width:500,
                defaultTextHeight:210,
                fn:function(btn,text){
                       //http processes
             }
    });

    //Set emptyString
    myMsg .textArea.setEmptyText('My Message');