Get password from get_sasl_hanlder

Hello, I am creating a module for custom authentication, but I don’t know how can I get the password submitted from user.

function provider.get_sasl_handler()
	return new_sasl(module.host, {
		plain = function(sasl, username, password, realm)
			module:log("info" , username, password, realm);
			local postdata = json.encode({ username = username, password = password });
			local result = http.request(post_url, postdata);
			return result == "true", true;
		end,
	});
end

In log file I found that password is always the name of host.

Please help me to know how can I get the password.