@Kumrvits
First of all your question is a bit unclear,
What do you want to do?
There are Three ways. if successfully logins
1) show an alert dialog box of javascript.
2) Show a popup window which can also be done through javascript.
3) using the below code you can show a dialog
Response.Write("<script type='text/javascript'>alert('Sucessfully login')</script>");
Response.Flush();
C# also provides to register client side script.
Page.ClientScript.RegisterStartupScript()
For this the below link will help you.
DevCurry.com
Hope you have got the answer