I wrote a Form Finisher in PHP and it is working as intended (extending AbstractFinisher). The only problem I have is that I need the get the form itself (preferably as a Node) in the Finisher, because I need to query a specific property of that form.
before using the fusion based form builder package, I transferred the form identifier in a hidden field to the finisher. I then used the CreateContentContextTrait to load the node again and access it’s properties.
Basically you build an array like this and then pass it to the form view helper:
Thanks a lot for the answer, I got it working now with your solution. There may be some security concerns with that (taking the ID from another form and changing the HTML) but I think it is okay for now.