To add a map it is very easy and it only requires a few minutes of your time!
Step 1: You want to find a map from the workshop or on Google, if you are downloading the map from the workshop please read:(https://steamcommunity.com/sharedfiles/filedetails/?id=865959209)
Once you have extracted or downloaded the file you will then need to login FTP (read: https://billing.kryptonnetworks.co.uk/index.php?rp=/knowledgebase/27/Connecting-To-FTP.html" if you don't know how to setup FTP) then grab the map which should have been extracted or downloaded.
Step 2: Drag and drop the map file into the maps folder.
Step 3: To get the map to start default when you start up the server, you will need to login to the game panel then go to the command line manager select "Custom Commandline" then click edit. In there you want to add the map name, for example, "rp_evocity_v33x" and then click save.
Once you have done that the map should launch every time that the server restarts.