You can combine and adapt the code for adding a transcript (Method 1) and a box around text to create an activity that has a click-to-reveal answer.

Click on the Source code Pro icon and scroll down to the point where you want to add an activity. Hit the Enter key on your keyboard to create a blank line in the code.

Copy the following code and paste it onto the blank line:

<div style="width: 600px; border: 1px solid black; border-radius: 5px; padding: 10px; margin: 10px;">
<h5>Activity</h5>
<p>In which UK city is The Open University headquartered?</p>
<details>
<summary><strong>Reveal answer</strong></summary>
<br>
<p>The answer is Milton Keynes.</p>
<hr>
</details>
</div>

Here's how the code above works onscreen: 

Change the text in the code to suit the text of your own activity.