Eloqua form with checklist and open text field

Hi,

I want to create a form with one question being a multiple checklist but with the option of adding options themselves so basically:

But I can't work out how to include the "Other (please specify) option so that a text box is included so they can write their own comment.

I can easily do it on other platforms (as you'll see from my example) so surely there must be a way to do it on Eloqua too.

Thanks.