There are two ways to use BlueSky or a compatible ATProto server to authenticate.
If you have a custom domain on BlueSky, you can enter it directly in the login form. You will be redirected to BlueSky to log in.
Alternatively, you can link to your BlueSky handle from your website, similar to how you would link to a GitHub or other external profile.
<a href="https://aaronpk.bsky.social" rel="me atproto">aaronpk.bsky.social</a>
Make sure you add rel="me atproto" to the link. You will also need to link back to your website in your BlueSky profile.