https://serverfault.com/questions/750902/how-to-use-lets-encrypt-dns-challenge-validation
sudo certbot certonly --manual --preferred-challenges dns
Certbot will then provide you instructions to manually update a TXT record for the domain in order to proceed with the validation.
Please deploy a DNS TXT record under the name
_acme-challenge.domain.com with the following value:
667drNmQL3vX6bu8YZlgy0wKNBlCny8yrjF1lSaUndc
Once this is deployed,
Press ENTER to continue
The TXT record should only needs the _acme-challenge part on namecheap for example.
To check the TXT record you can use the dig command
dig -t TXT _acme-challenge.domain.com