Select Tools -> Sign Message.
Copy your BTC address and paste it in the field "Address".
Enter the e-mail you registered with the BitStock exchange in the field "Message" (note: Mind the letter case).
If you have set the password for your wallet, it is necessary to enter it in the upper part "Wallet password".
Confirm by clicking on the button "Sign Message".
The electronic signature is shown afterwards, mark it and copy it to BitStock while adding new address.