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Use a Custom Webhook notification block when you want to send notifications to services that do not have a dedicated Prefect block. This guide shows a Telegram example that uses the Telegram Bot API.

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Telegram via Custom Webhook

Create a Custom Webhook notification block with the Telegram Bot API endpoint and payload. Example Custom Webhook configuration (UI JSON): 
{
  "name": "telegram",
  "url": "https://api.telegram.org/bot{{telegram_bot_token}}/sendMessage",
  "method": "POST",
  "json_data": {
    "chat_id": "{{telegram_chat_id}}",
    "text": "{{subject}}\n\n{{body}}",
    "parse_mode": "Markdown"
  },
  "secrets": {
    "telegram_bot_token": "<your-bot-token>",
    "telegram_chat_id": "<your-chat-id>"
  }
}
Notes:
  • Use the secrets field to store your bot token and chat ID, then reference them in the URL and payload.
  • parse_mode is optional; remove it if you do not want Markdown formatting.
Alternatively, create the block in Python: 
from prefect.blocks.notifications import CustomWebhookNotificationBlock

bot_token = "123456789:ABCdefGHIjklMNOpqrsTUVwxyz"
chat_id = "987654321"

telegram_block = CustomWebhookNotificationBlock(
    name="telegram",
    url=f"https://api.telegram.org/bot{bot_token}/sendMessage",
    method="POST",
    json_data={
        "chat_id": chat_id,
        "text": "{{ subject }}\n\n{{ body }}",
        "parse_mode": "Markdown",
    },
)

telegram_block.save("telegram-alert", overwrite=True)
Once saved, this block appears in the UI notification options. When the automation fires, Prefect populates {{ subject }} and {{ body }} with notification details.