How to make custom field (defined as function) in StackedInline hidden?

[Mirek Zvolsky]

Django 2.1.

I have this code:

class AnswerInline(admin.StackedInline):
    model = Answer
    form = AnswerForm
    fields = ("question_answer_type", "answer_plain")
    readonly_fields = ("question_answer_type",)

def question_answer_type(self, row):
    return row.question.answer_type

This works and I have new field "question_answer_type" in inline instances.

Now I want to make this field hidden. (I want use its content in JavaScript.)

I know there is forms.widgets.HiddenInput (or forms.HiddenInput ???) which I should probably use. But how to use it?