Hi @Frank Dombrowsky,
I'm still on 3.9.0, but there, you have to open the portal with this URL:
http://<your-jira-url>/servicedesk/customer/portals?customize=true
and there you can edit the Announcement Message.
If you look at the portal page, you can see a "Customize"-Button in the top right corner. This also opens the configuration form.
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