How can I hide answer box (in multi line text entry) until inserted video is fully watched?

Here is how our survey should look like:

1. We show different videos to our participants and after watching the video they have to answer a question.
2. Every video + question is on a new page.
3. Participants must watch the whole video, the question and the next button only appears after the whole video has been played.
4. Participants can start to write their answer only after the video is watched + question is displayed, since we want to make sure nobody's trying to cheat (=answering without watching the video).

We managed to hide the next button until video is completed, we managed to hide the question until video is completed, however we cannot hide the answer box (we use multiline text entry, with the video inserted in the question box).

What should we do to make it work? It is importan to have the video and the answer box in one question, because we will have to use randomization where every video has its own question and has to stay together.