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 record to the domain.

This verification method consists of 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 on the verification process.
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The txt record content will usually look something like this: 

Step 1: Choose DNS (TXT) as the method for verifying the ownership of your Target
​Step 2: Copy the text contents (beginning with 'Probely='), and insert them as a txt record using DNS.
​Step 3: Click "VERIFY", and start scanning!

DNS records might take some time to get updated across all DNS servers. This shouldn't take more than 48 hours though.

If your FQDN is a CNAME this verification method will not work. In that case, you'll need to use the file verification, the DNS CNAME verification, the Meta tag verification, or message us for manual verification. This is a limitation of the DNS itself, which does not allow for TXT records in CNAMEs.