These are three main requirements to become a Fatui Harbinger and sign up for harbinger position to serve bronya the tsaritsa:
1. Jobber (get btfo by practically everyone in teyvat)
2. Weak and easy to get btfo
3. Didnt do anything in both archon quest and your own story quest
If anyone have those skillsets they can apply for fatui harbinger position to pierro, bonus point if you have a shit and replacable kit that are useless for future region