How do I add a subdomain to a domain registered at IONOS pointing at Netlify nameservers?

I have tried changing the CNAME and this has not worked. The responding site is only text saying “Not Found”.

My domain name is chuteoff, and I want app.chuteoff to link to a Heroku app

What is the process to add the subdomain to this domain name? I will do anything

which custom domain and subdomain is this regarding?, .net?