If you confuse the two, you might think Cyberduck needs a key. They are different applications with different licensing models. Mountain Duck is proprietary, not open source. Cyberduck remains free.
You can drag and drop the license file onto the Cyberduck application icon or move it into the Application Support folder on macOS. Registration Key vs. Mountain Duck Cyberduck Registration Key
If you do not have a key, you simply click "Close" or "Later." The software continues to work 100% perfectly. If you confuse the two, you might think
The developer now directs all users to the free download from the website and encourages voluntary donations instead. Cyberduck remains free
(minimum $10 USD) to support the development of the application. Cyberduck Help Purpose and Benefits Remove Donation Prompts
: The key is typically sent via email as a file named yourname.cyberducklicense .