Before you can use a custom map on your Rust service make sure that the map you are trying to use is saved as ".map" and that the file is uploaded to a site where it can be directly downloaded from.
Once you have the direct-download link for your custom map, place it in the "Level URL" setting option found under the "Advanced Tab" - once you have done this. Click "Save & Restart" when the server comes back online you are now using your custom map.
Some maps may require other mods or extras be downloaded and added to the server, most of the time following a tutorial on how to do so, and where to place them. For these, you will want to use the "SFTP Tab."