If you are having a "Failed" error while creating your hostname, make sure that you have implemented DNS records.

You should have created a CNAME record in your DNS as it is given in the custom hostname setup and you should use it as DNS only.

You should change your settings to DNS Only from Proxied by clicking on the yellow cloud in the Cloudflare panel. After that, you can go back and verify your hostname again.
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If you're still getting the same error, please write to us.