For mail delivery, a DMARC record is an important record for your domain to help fight spam. For more information about the DMARC record and why it is used, please see this article: What is DMARC?
To create a DMARC record, log in to either your WHM or the cPanel account you want to add a DMARC record for and access the DNS Zone Manager.
With the DNS Zone Manager open, click the "Manage" button next to the domain you want to add a DMARC record to; this will show all of the active DNS for this domain.
Click the arrow next to the "Add Record" button to bring up a dropdown, and select DMARC from this list. cPanel/WHM includes an editor specifically for the DMARC record to assist you in creating this record with the options you wish to have.
Once you have configured your DMARC record with the configuration you wish, then press the "Add Record" button; this will add the DMARC record to your DNS.