Import the repository
In the sidebar, click the plus next to Projects. Pick a repository and import it.

If the repository is missing from the dropdown, the app is not installed on it. Use the configure link, then reopen the dialog.
Importing does three things: it records the repository and its default branch, it
registers a push webhook, and it pulls every open pull request in as a session with
status under-review. A repository with twenty open PRs arrives with a full board.