This is not built yet. Nothing on this page describes something you can switch on today.
A custom notification currently has two destinations, Discord and your own webhook endpoint. Telegram would be a third: the same rules, the same live summary of what they mean, delivered to a Telegram chat instead.
Until it exists, the way to get matches into Telegram is a webhook notification pointed at something of your own that forwards them on. That is documented in full, signature scheme included, under webhook notifications.
This page will be filled in when the channel ships, and no earlier. If it matters to you, say so through support: what people ask for is what gets built first.