...
With "Keyword", your users can easily jump to sub flows by typing in keywords rather than being lost in menus or buttons.
...
go "Automation" from the sidebar
click "Keywords"
for "default reply", see explaination below
click "+ New Keyword"
choose a condition from "is", "contains" or "starts with"
put one or more than one keyword, separated by only commas, no space allowed
choose a sub flow to send
Default Reply
"Default Reply" is sent when the bot doesn't know what to reply. When the active button is off or no sub flow was chosen for "Default Reply", the main flow will be sent.
...
Try Google Dialogflow Integration! Check how to setup Dialogflow.