Validating the ownership of a target through DNS doesn't involve making changes on the website. You validate the ownership of a target by simply adding a txt record to the domain.
This verification method consists on proving that you have control over the DNS records that point to your website. To do so, create a TXT record with the same FQDN/hostname as your target with the content given to you at the "Add Target" page.
The txt record content will usually looks something like this:
Step 1 : Choose DNS as the method for validating the ownership of your Target
Step 2: Copy the text contents highlighted above (beginning with 'Probely='), and insert them as a txt record using DNS.
Step 3: Click "Verify", and start scanning!
Common Issues and Mistakes
It might take some time
DNS records might take some time to get updated across all DNS servers. This shouldn't take more than 48 hours though.
CNAME FQDNs and txt records
If your FQDN is a CNAME this verification method will not work. In that case, you either need to use the file verification or message us for a manual verification. This is a limitation of the DNS itself, which does not allow for TXT records in CNAMEs.