sorry for the sarcasm but a big fan of copy and paste 
http://letsencrypt.readthedocs.io/en/latest/using.html#certbot-commands
–preferred-challenges PREF_CHALLS
A sorted, comma delimited list of the preferred
challenge to use during authorization with the most
preferred challenge listed first (Eg, “dns” or “tls-
sni-01,http,dns”). Not all plugins support all
challenges. See
https://certbot.eff.org/docs/using.html#plugins for
details. ACME Challenges are versioned, but if you
pick “http” rather than “http-01”, Certbot will select
the latest version automatically. (default: )
the manual even gives you the syntax 
below is how i usually do mine
