You cannot, and that is the honest answer rather than a step you have missed. Squarespace serves its own robots.txt as a system route. Code injection does not reach it and a URL redirect does not override it.
What to do instead
Start by reading what your robots.txt actually says, because on Squarespace it is usually not the problem.
Open
yourdomain.com/robots.txtin a browser and read it. If none of the four answer crawlers is named with a blanket disallow, the file is not what is blocking you.Read the technical detail on the finding. It says whether each crawler was refused by robots.txt or turned away by a firewall. On Squarespace it is far more often the second.
If it is the file, contact Squarespace support with the crawler names. It is their file, and asking is the only lever you have.
If it is a firewall, the block is between your visitor and Squarespace. Check anything you have put in front of the site yourself, such as Cloudflare.
Squarespace and AI crawlers
Squarespace has settings governing how AI crawlers may use your content, and they sit with the other search settings rather than in a file. Look through the SEO and AI visibility panel and read what is switched on. A setting there is more likely to be the cause than anything you have done.
If your screens do not look like this, the wording has moved rather than the setting. Search your platform for the words in bold above. If you still cannot find it, send us the scan link and we will tell you where it is on your setup.