Custom Headers possible? Authentication Problem with Jira Automation

We want to use Incoming Webhooks from Personio in our Jira-based Workflows.

Unfortunately, they do not seem to be arriving, most probably because of the authentication header not being compatible.

Atlassian Documentation:

Enter the URL and secret into your connected application, and add a new HTTP header with the name X-Automation-Webhook-Token. The method to do this can vary between applications, so you may need to check what the instructions are for your application. If your application does not support custom HTTP headers, you can instead insert a slash at the end of the URL and add the secret after this. For example, https://URL/SECRET. This will allow you to bypass the need for a HTTP header. However, we recommend using the header if possible, as it provides more security for your secret.

What can we do to make it work?

Thank you!