Join an empty official DDNet server, and vote any map you want in Escape -> Call Vote, or use "/map" in chat to search for a map. Join a team using "/team 23" in chat, you can /lock the team and /invite others to join it too. This means others can watch you play, and you get official ranks when you finish the map.
Use a public service to host a public server, like Trashmap: https://trashmap.ddnet.org/ You can upload any map you want, but you won't get official ranks this way.
