[2023] How to play modded swarms in unmodded server?

I was playing last night and I joined a game that had an automatic message in chat when I dropped in: "Welcome [my username], in case you missed the lobby title we are playing hazard 5 with double enemies today. Have fun!". It was indeed fun. The host offered to change the haz level between missions and I got to try haz 6 next as well. Now, the only thing: this server was unmodded (I have modded lobbies filtered out since 90+% of them are only using mods to make game easier). How can I replicate this myself, have game with harder swarms and not show modded?