How to get the ID of a Steam Workshop map for Team Fortress 2
Every map published on the Steam Workshop has a unique numeric identifier (its ID). Here is how to find it for a custom Team Fortress 2 map.
Browse the community maps published on the Steam Workshop here: https://steamcommunity.com/app/440/workshop/
Open the map's Workshop page: the URL looks like https://steamcommunity.com/sharedfiles/filedetails/?id=XXXXXXXXX. The number after "?id=" is the map ID.
Tip: if the ID does not clearly appear in the URL (Steam app, some browsers...), click the "Share" button on the Workshop page — the displayed link also contains the ID.
Important: the map must be set to "Public" visibility on Steam. A "Hidden"/"Friends only" map can never be downloaded. Check the visibility from the map's Steam page ("Change visibility").