Webflow doesn’t easily support this without using “something” external unfortunately.
I had another domain (with a different host, outside of webflow) which I just uploaded the ads.txt file to via an FTP client, and then referenced it in Webflow’s 301 redirects section of the settings page. If you own another domain, I think this is by far the easiest way of doing it. If you don’t own a second domain, perhaps one of your friends does? If they aren’t using that domain for Google ads, they could host the file for you at the root of their site and then you’d just add a 301 redirect into Webflow of:
/ads.txt --> https://www.myfriendsdomain.com/ads.txt
Alternatively, here’s the walkthrough I got from their support team. It will require you to purchase CDN storage; Amazon has an offer for 12 months free. But to be honest, given you can get a domain from a webhost like GoDaddy for 0.99 pence/cents a year and the fact that it is SO MUCH SIMPLER to just upload the file and then point Webflow at it, I’d recommend doing that instead:
We do not yet have the ability to host a file on the root of the domain, only the domain redirect option is going to work. To do this will require some external cdn storage where you can map a subdomain like cdn.example.com at the external storage, then upload the ads.txt file to that storage.
After external storage is setup a 301 redirect can be setup to redirect the example.com/ads.txt file that google is expecting to cdn.example.com/ads.txt
I made a video to help explain these options: https://cl.ly/3fd715954367
Here is an article on creating a cdn using cdn storage:https://css-tricks.com/adding-a-cdn-to-your-website/
Here is an article using 301 redirects: https://university.webflow.com/article/intro-to-301-redirects
The fact that Webflow doesn’t allow basic file hosting with direct (rather than relative) file paths seems staggering to me, but there are a lot of features that the CMS is missing in general. Hope this is of use!