By following this link, you can see how to create an API key in Billy, for integrating with Salary.

https://www.billy.dk/support/article/saadan-opretter-du-en-api-noegle-adgangsnoegle/

You start the integration by going to ''Integrationer'', then going to ''Regnskab'', then pressing ''Skift regnskabsintegration'' and then follow the simple steps.