As a Premium subscriber, you're able to manually add questions to your Challenge Bank in case you want to work on them later. To do this, click the "Add this Question to Challenge Bank" link.
When the question is added, you'll notice that the counter on the Challenge Bank button in the sidebar will change.
Note: if the question you're trying to add is already in your Challenge Bank, it will not be added again and the counter on the button will not change.
Your Challenge Bank is available here: https://cna.plus/challenge-bank/