Hi! I'm gonna show you how you can send a message (WordPress post) to a Telegram channel. All you need is a public Telegram channel and a bot. The last one you can create via BotFather in Telegram.

Steps:

1. Create a public channel and a Telegram BOT (via BotFather).
2. Remember the bot token.
3. Add the bot to administrators of previously created channel.

At the final step you should add the following code to your functions.php:

function telegram_send_message( $new_status, $old_status, $post ) {
  if( $new_status == 'publish' &&  $old_status != 'publish' && $post->post_type == 'post') {
    $apiToken = "TOKEN";
    $data = [
      'chat_id' => '@channel_name',
      'text' => "\nRead more: " . get_permalink($post->ID)
    ];
   $response = file_get_contents("https://api.telegram.org/bot$apiToken/sendMessage?" . http_build_query($data) );
  }
}
add_action( 'transition_post_status', 'telegram_send_message', 10, 3 );

Note: Do remember to replace TOKEN with your actual bot token and channel_name with the name of the channel.

Now, all the new posts will be also published in your Telegram channel.