To begin with you need to register/login to developer.paypla.com
After then you need to create various test accounts.
Then either you can login to https://www.sandbox.paypal.com or by click on ‘ Enter sandbox...
Ideally you should see a big list when you mouse-over profile menu-item.
Or you see a big list when you click profile menu-item
But in some case you see like this
The reason of this variation is that your account type is...
I realized that there is no query string at all. paypal simply forward to the redirect URL.
I started using sandbox. I made sure that auto return is enabled. and also PDT is enabled. still I am redirected to a plain URL.
finally I read this...