One of the features of smash.gg online tournaments is the ability to only allow your twitch subscribers to enter!
To do so, you'll just need to make a custom field under registration settings.
When you click on "Add Custom Field" a drop-down will appear and you will need to fill out the details. Make sure to save once you're done!
And that's it! Now only people who are subbed to your twitch channel may register! Make sure your subscribers know to link their twitch account to their smash.gg account or else they won't be able to register.