Before you configure a DMARC record, you must already have both SPF and DKIM records configured.
DMARC records are a security protocol that will log any fraudulent attempts to use your domain to send an email. DMARC records are stored in the form of a TXT record with the name ‘_dmarc’.
The value of the record is usually generated by your mail service provider. Or you can use this free tool to generate your own DMARC record.