So there's nothing I can do if the sticker was attached to the windshield?
You can't apply for a full refund if you attached the sticker, you can't remove it.
What is needed in order to apply for a full 2-year refund?
The full 2-year refund can only be applied if the registration was never used and it's been no more than 60 days since the date the registration was issued. A processing fee of 1$ is deducted from the refund amount.