https://forum.squarespace.com/topic/156310-is-it-possible-to-add-an-adstxt-file-for-adsense-to-root-of-domain/
I think I've found a solution! You have to go through several steps, including URL remapping.
Here's what I did:
1. Download the ads.txt file from Google AdSense.
2. On Squarespace, go to your Pages, and create a blank page under the "Not Linked" section.
3. On that page, create a text block and write anything you like. I wrote "test".
4. Highlight the text and click on the hyperlink icon. Once the small window pops up, click on the settings icon.
5. Go to the "File" section and upload the ads.txt file. Once uploaded, click on it to select it (a tick mark will appear next to it). Click apply then save the page.
8. Return to your page's dashboard and go to Settings --> Advanced --> URL Mappings.
9. In the box, type "/ads.txt -> /s/ads.txt 301". Save.
Et voila! You should be good to go. You can even delete the page you've created earlier - the file will still be saved on Squarespace. I think I've found a solution! You have to go through several steps, including URL remapping.
More info: https://www.graesen.com/blog/resolve-google-adsense-your-earnings-may-be-at-risk-error-for-squarespace
Check Seller json file from here, it contains seller info:
https://support.google.com/adsense/answer/9889911?hl=en
http://realtimebidding.google.com/sellers.json
google.com, pub-9293877793179476, DIRECT, f08c47fec0942fa0