Thank you for your help Chris. My goal is to get my WordPress install to appear to be its own instans, so I combine the great at writing at WordPress and communicating at the Fediverse.
My settings at yoursite.com/wp-admin/options-general.php?page=activitypub is the following:
* Post-Content set to Custom and:
<p>%title%</p>
%content%
<p>%permalink%</p>
So it should also include the content, not only the link
* Activity-Object-Type set to Note (default). Is there a better option here?
Everything works if I write directly in WordPress, but not when I use Indigenous to post on the site