Forward from non cloudflare domain

Hello, I have a plan and I do not know if you can successfully implement via cloudflare, we did not succeed in the setup.
We have the domain example.cz which has DNS at Amazon AWS and we cannot move it. We need to make a DNS record of app2.example.com and send it to cloudwlare on the domain app.cdn-example.com - it has DNS in cloudflare and we have a functional load-balancer on app.cdn-example.cz.
app.cdn-example.com works fine for us, but how do we direct app2.example.com to app.cdn-example? via CNAME we have a DNS record for ping or diq command, but not for http://app2.example.com I get an error:
Error 1001 Ray ID: 549a5bb9e9177bd0 • 2019 - 12 - 23 12:32:27 UTC
DNS resolution error

I am not sure, you mentioned quite a few domains here, but in order to use a domain on Cloudflare, that domain needs to be fully added to Cloudflare.