You can view the activity of a branch of a repository connected with a GitHub webhook. To do this, select an item and open the Links tab.
When you expand a linked branch, you see its pull requests and its commits that aren't on the default branch yet. This comes from GitHub every time you open it.
Select Link to this item next to a pull request or commit to link it. Things that are already linked aren't shown twice, and pull requests from forks will not be shown.
If your admin has set up incoming webhooks, then new commits and pull requests are linked to the item automatically a few seconds after they're pushed or opened. This works for branches created with Create branch, and for branches named p4plan/<item ID>... that are linked to the item.
Automatic links are added as the user who created the webhook, and appear in the item's history. An item can have at most 200 automatic GitHub links. All github.com links on the item count.