You can find your Hetzner API keys within the Hetzner Cloud control panel. Head to Hetzner.com, login to the Cloud section with your account details.
If you have a project already, click into that project. Alternatively, create your first project and then click into it once created.
Hover the left menu and find the "key" icon that should be labelled "Security", click into this and a tabbed page should appear showing your SSH Keys as default. Along the top, should be other tabs for API Tokens, Certificates, Members.
Click into API Tokens, and then click the "Generate API Token" button. A modal window should appear, give the token a description like "ServerAuth API" so you know what the token is for and select the "Read & Write" option for permissions.
Press "Generate API Token", the modal window should change and you'll be presented with a code that looks similiar to
Copy this and save it to your Hetzner server provider within ServerAuth.