To receive the Demo Tester badge (if you have not received it already), you’ll need to provide proof of ownership.
Post some sort of proof of ownership (e.g. the private message in which you received the key, your product key activation license, your Steam Library, an identifiable video, et cetera…)
Here are visualizations for two of those methods:
The other badges recently added (“ID Badge” and the three for being a top poster) are automatically distributed daily.