1. Create a Slack app
2. Enable the Bot user, add chat:write and emoji:read in Permissions>Scopes, and generate an OAuth token for it
3. 
4. Go back to the Slack App config and click on Event Subscriptions
5. Enable and paste your app's URL in the Request URL field like so: https://yourapp.ondigitalocean.app/slack/events
6. Subscribe to the bot event emoji_changed