NOTE: Below solution is supported by Capture One 10 and later.
Make sure you have another device that has access to internet if your computer does not.
1) When Prompted select your version of Capture One. One the Activation screen select "Manual Activation". Type in your Key and you will be given a 24 digiti Registration Key.
2) Use another device that is connected to the internet. Go to www.phaseone.com and login to "Login".
3) In My Pages click "Software License"
4) Scroll to the bottom and locate the "OFFLINE ACTIVATION" section. Click the "Activate Capture One Pro" button.
5) Enter the 24 digit registration key from step 1 above then click the blue "Generate Activation Key" button.
6) A new 24 digit Registration key will be generated and visible on the screen in your browser
7) Enter the 24-digit key from your browser back into the activation screen of Capture One and hit the "Activate" button. Done.