for a windows one generally the map is in the server folder, atleast on my local one, but there should be a spot i would assume to add a level url command, its kind of tricky to explain. and once again i have zero idea where anything is on linux, but most providers have a specific area you can add custom maps or boot up settings that include something like this:
RustDedicated.exe -batchmode +server.port 28015 +level.url "Procedural Map" +server.seed 1234 +server.worldsize 4000 +server.maxplayers 10 +server.hostname "Name of Server as Shown on the Client Server List" +server.description "Description shown on server connection window." +server.url "http://yourwebsite.com" +server.headerimage "http://yourwebsite.com/serverimage.jpg" +server.identity "server1" +rcon.port 28016 +rcon.password letmein +rcon.web 1
if it has a area like this you should be able to use either their site or drop box, if its dropbox copy the drop link and paste it into "Procedural Map" so it looks like "custom map name" and will end in a 0, change the zero to a 1