I increased your trust level to “3 – Regular”. You will be able to tag any own or wiki topic now. And sure, you can tag this topic for testing.
(Note, not for now but for 6 months to two years from now: I’m not happy with wildly tagging things in Discourse. See the recent discussion with @hugi in the webkit strategy topic. So consider this tagging mechanism ok for now, but not the final mechanism to select posts for the webkit. Link references from the webkit configuration to Discourse will be better.)