Adding DNS record in a DNS provider
Login to your DNS name server
Select the domain and go to the DNS record section
Copy the values from the custom domain page and add a new record
If you are using a domain with a subdomain, then the record type will be CNAME in most of the name servers
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If you are using the root domain as the custom domain, then the record type changes depending on your name server
Below are links to the help articles of popular DNS name servers.
Google domains: Create a CNAME record
Domain.com: How to update a CNAME
DreamHost: Adding a CNAME
Bluehost: What is a CNAME?
Hover: Managing DNS records
Dynadot: How do I create a CNAME?