The official Trosnoth web server runs on play.trosnoth.org. When you install Trosnoth, it is configured to use this server by default.
If you want to hear about (or organise) online matches or tournaments, you may want to join the Trosnothers Facebook group. Others in that group will be able to invite you to our Discord server. There's also an old Google group that doesn't get used much any more, but if you don't use Facebook, that Google group is a great way to ask about planned matches.