I am adding custom NS of my domain in DNS tab in Cloudflare but it always says
DNS Validation Error (Code: 1004): Invalid content for a NS record (Code: 9037)
This is my NS: clint.ns.Cloudflare.com
But when I add clint.Cloudflare.com it is added successfully. Please help me regarding this issue as I am currently facing DNS Modification Pending status
You shouldn’t have to add an NS record here. It’s already built in to your account.
The NS records need to be modified at your registrar, not within Cloudflare. For more information on switching your nameservers, please see the following article: https://support.cloudflare.com/hc/en-us/articles/205195708-Step-3-Change-your-domain-name-servers-to-Cloudflare
What if I am using Cloudflare as my registrar? There is no option to change name servers.
Correct. At this point, it’s pretty foolproof.