sorry for the sarcasm but a big fan of copy and paste 
http://letsencrypt.readthedocs.io/en/latest/using.html#certbot-commands
âpreferred-challenges PREF_CHALLS
A sorted, comma delimited list of the preferred
challenge to use during authorization with the most
preferred challenge listed first (Eg, âdnsâ or âtls-
sni-01,http,dnsâ). 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 âhttpâ rather than âhttp-01â, Certbot will select
the latest version automatically. (default: )
the manual even gives you the syntax 
below is how i usually do mine
