This is the root of the Power-up 👋.
Trello uses this to start your Power-up and show UI elements 👀.
1. Go to Power-up Admin and update the URL as appropriate
2. Enable the Capabilities that you want to expose in the Power-Up
3. Add the Power-up to a Board via the Add Power-Up Menu (Choose Custom Filter)
4. Use your Power-up as you code and see changes to the UI in near real-time!