Any RTV sealant should do
I've used blue one.
You really don't have to put much, a 1mm thick bead will seal more than enough!
Put the silicon along the small groove (it's a guideline, and prevent too much silicon from being crushed out), the bolt the TB to the plenum and clean the silicon that will have been pushed out (you can open the throttle plate by hand
/!\ but unplug the TB first! /!\)