Short.io allows creating short links straight from Slack using your own domain.
Follow the instruction below for shortening:
1. Go to your Short.io account.
2. Open User Menu.
3. Choose the "Integrations & API" tab.
4. Open the "Slack" tab.
5. Click "Connect."
7. Choose Allow.
8. Select a domain.
9. Confirm by clicking Select.
10. Slack integration is turned on.
11. Type “/shorten” in any Slack channel/conversation and paste a long link.
12. Send this message and the service will respond with a short URL.
Watch the video below. It will guide you on the installation and make the process clearer.