Create a "new text" with the HTML code of the last page.
Then insert a page before the last page. You won't be able to have a "next" button on the last page (because then a very last page would be necessary to be shown), therefore you need another page.
Then put the text from above plus you question on that new page. When respondents type something in the open-ended input, and click "next", the input will disappear and only the goodbye text will remain.