From Workshop Link[ | ]
In order to get the id of a workshop mod, go to its workshop site, and look at the last number of its URL. That's your mod's id.
So for https://steamcommunity.com/sharedfiles/filedetails/?id=1682700568, the ID would be 1682700568.
From modinfo.lua[ | ]
In case of a subscribed workshop mod, you can look into the modinfo.lua file that will be in that mod's directory. Open the file with a text editor, and look for a line like id = "1682700568",. That number is your ID (without the quotation marks).