Just enter any Bluesky username (the part after the @) in the box and hit the Go button or enter/return key.
Here's mine: https://jasoncoon.github.io/bsky-metrics/?username=evilgeniuslabs.org
The web app just makes a few public, anonymous API calls to Bluesky to get the profile and page through the full followers list, charting the date those followers joined Bluesky.
I initially set out to display a chart of follower counts by the date they followed. This data doesn't seem to be available. I realized what I was actually charting was the date a user's followers joined Bluesky, which I guess is interesting.
I may add more data as I explore the Bluesky APIs.