I think it is automatic. It starts crawling other instances starting from lemmy.ml, so it should appear soon, since your instance is listed under their lemmy.ml/instances. I don't know the periodicity of crawls though.

Here's the documentation https://join-lemmy.org/docs/en/administration/first_steps.html#inclusion-on-join-lemmyorg-instance-list

• Federate with at least one instance from the list
• Have a site description and icon
• Be patient and wait the site to be updated, there's no fixed schedule for that

My instance showed up after a day or two. The installation instructions say:

1. Federate with at least one instance from the list
2. Have a site description and icon
3. Be patient and wait the site to be updated, there's no fixed schedule for that

For (1) I made sure I subscribed to a community in lemmy.ml from my instance.

the crawler seems to run somewhen after midnight cest – maybe 1-3am

I can confirm that my instance is in the list now - it seems that it was in fact just a matter of waiting a bit.