Mod Maps, how to make them multiplayer?

In detail, how do you play a map you've made online in multiplayer?
And then please tell me why this isn't a simple setting. I've found some hacks that say they do it with no instruction on how to...and they don't make custom maps multiplayer.

Why in the world would a developer make a multiplayer game you can create content for but can't use the content you create on multiplayer?
I do not want whatever it is they're smoking.
I'd just like to make my maps multiplayer so I can play them with friends.

What would be really nice is some actual, GOOD, documentation and info on the map editor.

The Big Book of Mudrunner Map Making has an answer for how to play a mod map in multiplayer, although it's buried in Appendix A -> Playstyle Considerations -> Multiplayer -> Testing Multiplayer, and it applies only to mod maps published through Steam. Note that you can publish as "friends only" for testing purposes.

If you're instead asking how to make a map that supports multiplayer, every map implicitly supports multiplayer. The game duplicates trucks as needed to support extra players. More information is in that same Multiplayer section of the BBMMM.