When the user tries to browse through a website through local intranet like https://mypay he will not be asked for any user id and password . But when he tries to browse through internet using https://mypay.mycompany.com he will be asked for user id and password using a prompt box. How to implement this pop-up scenario when user is browsing the application through internet, in struts.
Well, I didn't get your question. In both the cases, internet or intranet, you need to get authenticated by submitting uer/password in order to access the web app. Also, https://mypay.mycompany.com is subdomain.
And login screen can be obtained by popup window or using simple page, its not Struts or any other framework dependent.
BTW, the login popup might be related to FORM based authentication.