If you've forgotten your password, click the Forgot Password link in the Sign-in window when you open Final Draft.
Enter the email address you used when you bought Final Draft 13 and click Next:
You will receive an email with a reset password key. Copy this key by selecting it all and clicking Command + C on Mac or Control + C on Windows:
Return to the Final Draft program and you will see this screen; paste the key into the box with Command + V on Mac or Control + V on Windows and create and enter a secure password:
Click Sign in and you should be able to open Final Draft.
NOTE that no one at Final Draft has access to your password. Be sure to write it down and keep it in a safe place.