The SQL REST API accepts SQL in a JSON document, executes it, and returns the results. For example:

POST /_sql?format=txt
    "query": "SELECT * FROM library ORDER BY page_count DESC LIMIT 5"

Which returns:

     author      |        name        |  page_count   | release_date
Peter F. Hamilton|Pandora's Star      |768            |2004-03-02T00:00:00.000Z
Vernor Vinge     |A Fire Upon the Deep|613            |1992-06-01T00:00:00.000Z
Frank Herbert    |Dune                |604            |1965-06-01T00:00:00.000Z
Alastair Reynolds|Revelation Space    |585            |2000-03-15T00:00:00.000Z
James S.A. Corey |Leviathan Wakes     |561            |2011-06-02T00:00:00.000Z

Using Kibana Console. If you are using Kibana Console (which is highly recommended), take advantage of the triple quotes """ when creating the query. This not only automatically escapes double quotes (") inside the query string but also support multi-line as shown below: console triple quotes