Here's the way 21A works (I've learned this the hard way).
1. Can you taste the licorice flavor in your beer? If it just tastes like an Oatmeal stout, enter it in 13C. If it has a noticeable licorice flavor that would be considered not-to-style in 13C, then enter it in 21A. If the licorice is hardly noticeable, just something that makes you go "hrm.... that's nice and interesting", stay in 13C, you'll place better. When you enter in 21A, that specialty ingredient is going to be something the judges are DEFINITELY looking for.
2. If you do enter it in 21A. State that you brewed an 'Oatmeal Stout with Star Anise (Star Anise imparts a licorice flavor)'.
Make sure to mention Star Anise tastes like licorice; don't assume the judges will know.
3. 21A is very much a category about balance. It's judged considering how well the base beer tastes according to style, and how well the specialty ingredient is incorporated. It should be present, without being completely overpowering.