Hi Vikash,
Are you looking for login screen customization for particular application, use SICF LOGIN screen customization:
Change logon screen of SAP web dynpro
Here, you can have your custom logic in login button's event handler.
If you want to have the authorization checks in WD application, please refer the below document
How to handle custom authorization checks in Web Dynpro ABAP - Part 1
Hope this helps you.
Regards,
Rama