Make sure you've completed the Configuring the server to download files from Workshop guide.

Steam Workshop map location in server and map names

Start the server and wait it to download Steam Workshop maps you added earlier. This can take several minutes. After the server has downloaded all the maps, stop the server and make sure that it is not running.

Navigate into server install location and go into folder ROGame\Cache\XXX\0\BrewedPC where XXX is one of the ServerSubscribedWorkshopItems you added earlier. For example (RO-BikiniAtoll):

ROGame\Cache\643383080\0\BrewedPC

Copy and paste the map file name (excluding the .rom extension)

To add custom maps to your map cycle:

Edit ROGame.ini, make sure bUseMapList=True

Find the line GameMapCycles=(Maps=("VNTE-HueCity","VNSU-AnLaoValley","VNSU-SongBe","VNTE-CuChi","VNTE-Hill937"),RoundLimits=(0,0,0,0,0))

Add the custom map's name you copied earlier to the end of the map list surrounded in speech marks "" with a comma prior. Also make sure to add an extra ,0 in the Round Limits.

It will look something like this:
GameMapCycles=(Maps=("VNTE-HueCity","VNSU-AnLaoValley","VNSU-SongBe","VNTE-CuChi","VNTE-Hill937","RO-BikiniAtoll"),RoundLimits=(0,0,0,0,0,0))

Save the file.

To get custom maps to show up in the change level section of web admin:

Admins will need to add a data store for the game in ROGame.ini. A data store looks like the following example:

[te-custommap ROUIDataProvider_MapInfo]
MapName=ro-bikiniatoll
FriendlyName=Custom Map
GameType=ROGame.ROGameInfoTerritories
PreviewImageMarkup=<Images:ui_textures.menus.HostGame.MapSelect_Community>
Description=Mamayev Kurgan
LoadMapMovie=LoadScreen_Mamayev
MapTips=TEMamayevTip2
MapType16=ROMT_InfantryOnly
MapType32=ROMT_InfantryOnly
MapType64=ROMT_InfantryOnly

Please note the use of lowercase in the map name and read the Known Issues to Avoid section for more information on the topic. This holds true for the mapcycle as well.