Thanks a lot! I ended up with an inline function in the html form fields, since when invoking a function that I defined in the JAVASCRIPT variable, I got errors stating that the function was not defined (yet). Not sure why that is, but maybe because the JAVASCRIPT content is inserted into the webpage after all the HTML is rendered and processed?
For anyone interested in this solution, a minimalistical HTML in my recipe looks like this:
<input key="{{_task_hash}}" type="checkbox" id="checkbox_is_sentence_issue" onchange="(function(checkbox_object){window.prodigy.update({ [checkbox_object.id]: checkbox_object.checked })})(this)" />
<label for="checkbox_is_sentence_issue">Sentence fragment or multiple sentences</label>