Thank you for your answer.
I found the articel you mentioned, it’s very good written.
Maybe I have a misconseption, but the article only explains how to add in old style block editor.
With the new beta site builder there is no “single.php” file anymore.
The templates are created in site builder, I can see the code, but it looks like:
<!-- wp:group {"layout":{"type":"constrained"}} -->
<div class="wp-block-group"><!-- wp:spacer {"height":"51px"} -->
<div style="height:51px" aria-hidden="true" class="wp-block-spacer"></div>
<!-- /wp:spacer -->
I am pretty shure I have to add the custom fields in that format, but I can’t find how