Even if the Webflow CMS field allows for uploading a TXT file (File field | Webflow University), the asset will be placed on a CDN and its URL won’t contain your domain, wich I guess won’t work for Apple to approve the integration.
Apple wants to make sure you’re in control of your domain name.
So maybe try this:
create a simple site on a server you can access with FTP or SSH and place your TXT file on it
go to your registrar and edit the Zone DNS, replacing Webflow DNS by ones that point to your newly created site
make apple approve your site
set the DNS back to Webflow
As long as Apple doesn’t require for the TXT file to remain on the server after approval — which I think isn’t required — it can work. But for sure there is no way to have your TXT file under your domain on the Webflow site.