Hello,
It is resolved by using the following code provided by @Tyson
require_once "path/to/license.php";
$server_url = "https://domain.com/path_to_blesta/plugin/license_manager/validate/";
$license_key = "YOUR LICENSE KEY"; // The license key
$public_key = null; // The client's public key (if they have one)
$shared_secret = "YOUR SHARED SECRET"; // A random shared secret value that exists for this Licese Module product
$path_to_phpseclib = dirname(__FILE__) . DIRECTORY_SEPARATOR . "phpseclib". DIRECTORY_SEPARATOR; // The path to the phpseclib library
$license = new License($path_to_phpseclib);
$license_manager = $license->getManager();
$license_manager->setLicenseServerUrl($server_url);
$license_manager->setKeys($license_key, $public_key, $shared_secret);
// Get the public key
$public_key = $license_manager->requestKey();
var_dump($public_key);