In the Hashtag section on the Overview screen of your Photo/Video Contest’s dashboard, click the toggle switch to enable hashtag entry from Twitter.
Then type in the required hashtag or hashtags for entry (you can require just one, multiple, or add separate optional hashtags).
Now, let fans know that they can enter by tweeting and including your required hashtag(s) in the caption (the entrant’s profile must be public as well).
You can also use the Twitter Retweet picker or Twitter Follower picker in the Social section of your dashboard to select winners from those groups. More on that feature here.