Create a file that's named ad-selection-attestations.json, and host the file at the top-level domain, in the /.well-known/ directory. For example:

https://contoso.example/.well-known/ad-selection-attestations.json

The ad-selection-attestations.json file must be published within 30 days of receiving the OT token. Hosting this JSON file is required, in order to complete your attestation and allow your code to access the Ad Selection API, to test the Ad Selection API with supported Microsoft Edge clients.

View an example ad-selection-attestations.json file with documentation.