Adding robots.txt only to a custom subdomain I have, but not the root domain

Hello, I am posting to see if anyone knows if what I want to do is possible…

I have my site published on a main domain, but also a totally different domain as a sub-domain.

So for example: &

I just recently realized that the was searchable by google, and I’d like to fix that asap without having to unpublish. I just want it to act like the webflow url that includes the robots.txt.

Does anyone know a way to do this? Much appreciated for anyones time reading through this!

Thank you!

Here is my site Read-Only: LINK
(how to share your site Read-Only link)