Do not put # on the room name
Use '-' to send a private message e.g.
echo "hey greg" | curl -d @- p2ir.cf/irc.server.com/-greg
To send multiple lines use --data-binary curl argument
printf "hey greg\nhow are you\nbye" | curl --data-binary @- p2ir.cf/irc.server.com/room
If you want another nick set the header
printf "hey greg\nhow are you\nbye" | curl -H "NICK: samuel" --data-binary @- p2ir.cf/irc.server.com/room
For more info and for using this on github webhooks take a look at https://github.com/matheusfillipe/p2irc