Forum ID's can be found by navigating to a forum URL, either on the web portal or in the admin console.
On the web portal, navigate to the forum you wish to obtain the ID for. The URL will look something like this:
The ID is what comes after /forums/, so in this case, it is
In the admin console, a forum ID can be found by clicking into the forum from the "ideas" area in the left sidebar.
The URL will look something like:
The number after /forum= is the ID,