Step 1 — Declare a developer website
For iOS: App Store Connect → App Information → Marketing URL. For Android: Play Console → Store presence → Store listing → Website. The URL in either field must respond with HTTP 200 and serve app-ads.txt at the root.
Step 2 — Publish app-ads.txt at the root
If your declared site is example.com, your file lives at example.com/app-ads.txt. If it is example.com/myapp, the file goes at example.com/app-ads.txt regardless — it is always root-of-domain.
Step 3 — Wait 24 hours and verify
AdMob → Apps → app-ads.txt status. Each app's status flips from "Not found" to "Authorised" within 24 hours of the file going live. AppLovin and IronSource expose similar verification dashboards.