Use MultiMC or Prism Launcher to connect - ensure you’re using Beta 1.7.3, not the release version. (Toggle the “betas” filter and select “b1.7.3” when creating your instance)
Once you are whitelisted, the server is accessible at oldschoolmc.live:25566
Whitelist application is here (After submitting, you should be whitelisted within 12-24 hours, if not sooner)
Rules are on the website
I got my friend to submit an application as well - we’ll probably be basing together, very excited to play old school MC with him :D