To set up an AdSense ads.txt file on Squarespace, you'll need to download the file from Google AdSense, create a blank page on your Squarespace site, upload the ads.txt file to that page, and then use URL mappings to redirect the ads.txt file to the root of your domain. 
Here's a more detailed breakdown:
1. Download the ads.txt file from AdSense:
- Log into your AdSense account.
 - Navigate to the "Sites" section and locate your Squarespace website.
 - Download the ads.txt file associated with your site. 
 
2. Create a blank page on Squarespace:
- Go to "Pages" in your Squarespace dashboard.
 - Create a new page under the "Not Linked" section.
 - You can leave this page blank, as it will only be used for the file upload. 
 
3. Upload the ads.txt file:
- On the blank page, add a text block and write any text you want (e.g., "ads.txt").
 - Highlight the text and click the hyperlink icon.
 - Click the settings icon (gear icon).
 - Navigate to the "File" section.
 - Upload the ads.txt file you downloaded from AdSense.
 - Select the uploaded ads.txt file and click "Apply".
 - Save the page. 
 
4. Use URL mappings:
Go to your page's dashboard, Navigate to "Settings" > "Advanced" > "URL Mappings", Add the following URL mapping: /ads.txt -> /s/ads.txt 301, and Click "Save". 
5. Test the ads.txt file:
Important Notes:
- Ensure your ads.txt file is formatted correctly as specified by the IAB Tech Lab. 
 - You can delete the blank page you created earlier, as the file will still be saved on Squarespace. 
 - If you encounter any issues, consider contacting Squarespace Support or Google AdSense support for assistance.