App Platform: Set subdomain to redirect to folder on main domain

I’m using app platform for a site, and I want to set it up to redirect a subdomain to a folder of the main domain. e.g.:

foo.example.com => example.com/foo

how do I do this? I’ve already setup foo.example.com as a custom domain in my app, but I can’t figure out the next step. I tried to do it via CNAME, but that tells me that example.com/foo is an invalid alias