This is the answer from SO :

Another method would be to modify the droplet as it is created. With Digital ocean you can pass User Data, previously I have used this to configure servers automatically, here is an example.

$user_data = <<<EOD
#!/bin/bash

apt-get update 
apt-get -y install apache2 
apt-get -y install php5 
apt-get -y install php5-mysql 
apt-get -y install unzip 
service apache2 restart 
cd /var/www/html 
mkdir pack 
cd pack 
wget --user {$wgetUser} --password {$wgetPass} http://x.x.x.x/pack.tar.gz
tar -xvf pack.tar.gz 
php update.php
EOD;


    //Start of the droplet creation
    $data = array(
                    "name"=>"AutoRes".$humanProv.strtoupper($lang), 
                    "region"=>randomRegion(), 
                    "size"=>"512mb", 
                    "image"=>"ubuntu-14-04-x64",
                    "ssh_keys"=>$sshKey,
                    "backups"=>false,
                    "ipv6"=>false,
                    "user_data"=>$user_data,
                    "private_networking"=>null,
                    );

    $chDroplet = curl_init('https://api.digitalocean.com/v2/droplets');
    curl_setopt($chDroplet, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($chDroplet, CURLOPT_POSTFIELDS, json_encode($data)  );
    curl_setopt($chDroplet, CURLOPT_HTTPHEADER, array(
        'Authorization: Bearer '.$apiKey,
        'Content-Type: application/json',
        'Content-Length: ' . strlen(json_encode($data)),
    ));

Basically once the droplet is active it will run these commands and then download a tar.gz file from my server and execute it, you could potentially create update.php to call your server and therefore update it that the droplet is online.

So, with the User Data we can run a wget command to launch a webhook. With this solution we can have an “event based” notification.