Send a POST to http://api.steampowered.com/ISteamRemoteStorage/GetPublishedFileDetails/v1 with the following parameters:

* 'itemcount' : '1'
* ''publishedfileids[0]' : 'WORKSHOP MOD ID HERE'

If collection, send POST to http://api.steampowered.com/ISteamRemoteStorage/GetCollectionDetails/v0001/

Download URL in 'file_url'

Reference: https://greasyfork.org/en/scripts/5729-enhanced-steam-workshop-downloader/code