Hi 
I posted a more elaborate response regarding static json responses in the context of matrix servers before, which I highly recommend to you here 
But just for the record, you could use both backslashes \ or backticks `:
The Caddyfile Concept docs (hotlink to “Tokens and quotes”) state:
Quotes can be escaped if you need to use quotes in quoted tokens, too:
directive "\"abc def\""[…]
You can also use a backtick
`to quote tokens; these are convenient when tokens themselves contain double quotes, e.g. JSON text:directive `{"foo": "bar"}`