Install certbot first

Run following command for obtaining a wildcard certificate:

  certbot certonly --manual --preferred-challenges dns --agree-tos -d *.example.com

Then follow certbot instructions for adding TXT record to your domain.

Example of DNS bind9 record:

; Letsencrypt verification
 _acme-challenge.example.com TXT "xxxxxxxxxxx_xxxxxxxxxxxxx_xxxxxxxxxxxxxxxx"