Login to your Namecheap Account, choose 'Domain List' on the left and click on the 'Manage' button next to your domain:
Navigate to the 'Advanced DNS' tab from the top menu and click on the 'Add new record' button:
Select 'TXT Record' for 'Type'.
Host: Enter the host name for the TXT record. For example, leave the host blank for an SPF record, or enter the value _dmarc for a DMARC record.
TXT Value: The value you are setting as the destination of the Host.
TTL: Determine how long the server should cache information.
NOTE: The domain name itself should not be included in the Host field. It means that if you need to add the record for _dmarc.yourdomain.tld, only _dmarc is to be added as a Host value.
Create a free OnDMARC account to test your configuration.