Since Windowblinds will nag you to activate, here is what you need to do.
Open a web browser, go to temp-mail.org
Go to Windowblinds and click on "Start 30 day trial".
Enter the email temp-mail.org gives you.
Send the confirmation email, go back to your browser and there should be a confirmation mail.
Click on it, do as whatever it says, and now Windowblinds will start it's 30 day trial.

If the trial expires, uninstall Windowblinds and reinstall it again, repeat as everything I said.