Click on the tab "Receive" where the list of the addresses of your wallet.
Select the address you wish to add on BitStock and mark it.
Click on the right button and select the option "Sign Message".
Enter the e-mail you registered with the BitStock exchange in the field "Message".
Cofirm by clicking on the button "Sign" afterwards.
The electronic signature of the address is shown in the field "Signature", copy it afterwards to BitStock while adding new address.
Linux Command Line Signing Manual
Verification of existence of the electrum wallet and displaying the format of the message signing order:
electrum help signmessage
The result should look like this:
Signs a message with a key
Syntax: signmessage <address> <message>
For signing the message enter the following order:
electrum signmessage <address> <message>
where <address> is your BTC address and <message> is your e-mail address of the BitStock account