I am calling a third party URL from my application using window.showModalDialog

tfaData = window.showModalDialog(cdBaseURL+"?CALLTYPE=GET_TWOFACTOR_AUTH_PAGE&LCLANG="+langCode+"&USERID="+username,argArr,"dialogWidth:626px;dialogHeight:500px;status:No;help:No;resizable:No;scroll:No");
            //alert(tfaData);
            alert(tfaData.UserID);
            alert(tfaData.Errorcode);
            alert(tfaData.Errormsg);
            alert(tfaData.Token);

The problem is the third party URL is returning the required values but I am not able to read the values in tfaData. Below is the third party code

How can I modify my js file to read the response?