To create an integration between Salary and Billy, you first need to create an access key in Billy to set up the integration. 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/
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You start the integration by going to Integrations -> Bookkeeping -> Choose Change accounting integration -> Follow the steps

The integration is now setup.

Finally, you can change some settings, such as whether you want Salary to post the attachment directly or put it in for approval in the cash journal.

Select Advanced for more settings.