RE: Connect your Bot to the World - XML, JSON, Web Services, HTML Scraping, Twitter, Facebook, Telegram, Email

It should work the same. Try using the debug() to see what is occurring.

var a = conversation.getLast(#input, 1).input;
debug(a);
var s = a.substring(3,a.size());

The debug will be printed to the log in chat if you click on the "debug" checkbox while chatting and select FINE as the level.

Also try ensuring it is a string.

a = a.toString();
var s = a.substring(3, a.size());

For an HTTP POST request use,

(XML)
Http.postXML(url, xmlObject);

or, (JSON)
Http.postJSON(url, jsonObject);

or, (form params)
Http.postHTML(url, params);

See,
https://www.botlibre.com/manual-self.jsp#http