ShartyFarter from the documentation (https://plutonium.pw/docs/modding/loading-mods/#loading-custom-maps-mp) :
Once the map is installed, you must load a mod first. If you do not have a mod to go along with your custom map, you can load a dummy mod by opening the console and typing /fs_game mods/mp_custommaps; vid_restart followed by /map mp_yourmapname and it should load right up.
Dedicated servers running custom maps also need to have a mod loaded.