You can find your DigitalOcean API keys within the DigitalOcean control panel. Head to DigitalOcean.com, login with your login details and find the API link on the side bar menu.
Click to the "Tokens" tab, from here you can view all active Personal access tokens, create a new one or regenerate an existing token for security. You can also get to this page from this link.
Click to generate a new token, give it a name relevant to ServerAuth so you know what that API key is for, set an expiration time, grant it write access and finally click "Generate Token".
A new token will be added to the table and the token itself will be visible underneath the name you gave the token. It will look similar to
Copy this and save it to your DigitalOcean server provider within ServerAuth.
NOTE: When setting the expiration time, best practises dictate that you should set an expiration and refresh the token every few months to prevent unwanted access. However, please be aware that in doing so ServerAuth may not be able to access your DigitalOcean account and may not be able to perform all the actions required.