Free_text field at Checkboxes

I have a Contact 7 from and i want the last option from a chexbox question to be an open text field where users can fill in a personal answer when the answers are not there already.

So:

<p> Wat is het type object? [checkbox Type_object “Woning” “Schuur” “Kantoorpand” “Anders”]</p>

How can i make the last option “Anders” to be an open text field where users can fill in text themselves? How do i apply this:
free_text Append a free input text field to the last item.

Thanks in advance!

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