Hi @Vitaliy thanks for your detailed help.
I already was able to add additional data by a smarty template during step 3 submission metadata using the hook Templates::Submission::SubmissionMetadataForm::AdditionalMetadata.
It works fine.
But my issue right now is how to insert something into the metadata area right before Schedule for Publication.
I would like to add a map based on Javscript code at this point.
In case of the step 3 submission metadata it worked fine with the hook Templates::Submission::SubmissionMetadataForm::AdditionalMetadata::AdditionalMetadata and I was able to extend the original template with a smarty template (geoOJS/geoOJSPlugin.inc.php at master · tnier01/geoOJS · GitHub), but how can I do it here?
I already know how I can insert in general something into this template by the hook Template::Workflow::Publication (lower left corner) but I want to insert something in the Metdata area. It is not a further form I want add, which is possible by the Hook Form::config::before of course. But I want to add a map based on Javscript Code. Is there a way how I can insert something at this point, a map or generally a html element?