Click on "Add a question" or "Edit" an existing question

Scroll to the bottom and click on "Select topic(s)" dropdown

At the bottom of the dropdown, select "Create a new topic/subtopic"

Type in the topic or subtopic name

If you want to create a subtopic, click on "nest as subtopic under existing topic"

Select the parent topic to group the subtopic under

Click on "Create" and you're done!