This problem was solved by Kris, whom I contacted by e-mail. I still have other unresolved issues, but I will try to solve them myself (by reading/watching the plugin manual). Thank you for pointing out the problem: Label is empty and that is why you cannot set up any other options as selected or a whole field as required. Please remove those empty fields and use placeholders (Placeholder). With that, you will be able to use the “Apply” button.
Thanks again to everyone, especially Kris Tomczyk for the tip.
I’m closing the topic.
