dig retool.examplesubdomain.com CNAME +noall +answer
; <<>> DiG 9.10.6 <<>> retool.examplesubdomain.com CNAME +noall +answer
;; global options: +cmd
retool.examplesubdomain.com. 3600 IN CNAME examplesubdomain.retool.com.
try to point it to custom-domain.retool.com instead of examplesubdomain.com – Configure a custom domain for cloud organizations | Retool Docs