You can create a DMARC record for your domain name by checking in your hosting providers control panel service.
If your hosted with us login to your cPanel account and go to the Email Deliverability option and follow the steps.
Alternatively you can go to DMARC Record Generator - Create DMARC DNS Records - MxToolbox
this will allow you to create a valid dmarc record which can then be added to your domain names DNS setup as a TXT entry
