Creating Custom DNS servers

We are about to add a domain to a Business plan, and want to create custom DNS servers.
Adding NS01.xxx.xxx,NS02.xxx.xxx and OK “check box” but nothing happens.
I want to use my Registrar DNS servers and only add a glue record for a subdomain to Cloudflare.

Anyone who have an idea if this is possible and if it is how to?

Thanks

Are you following the steps here, https://support.cloudflare.com/hc/en-us/articles/200169006-Setting-up-Custom-Nameservers-at-Cloudflare?