A Telegram deep link is a t.me/<bot_username>?start=<payload> URL that opens a chat with a bot and automatically passes a short payload to it — Telegram delivers the payload as the argument of the /start command the moment the user opens the link. The payload is limited to 64 characters using only letters, digits, underscores, and hyphens, so operators typically encode anything more complex with base64url. Because the payload survives the click, a deep link doubles as a lightweight attribution tag: encode which channel, post, or campaign the link was placed in, and the bot can read that context back the instant someone starts talking to it.
Every bot has a fixed https://t.me/<bot_username> address that opens a conversation with it, and appending ?start=<payload> attaches data to that link without requiring the user to type anything. The moment someone follows it, the bot receives a normal /start message with the payload as its argument — for example a link ending in ?start=airplane arrives as the text /start airplane. Nothing about the mechanism requires the bot to have ever seen that user before; the payload is the entire handshake.
A private-chat start parameter and a group startgroup parameter are not interchangeable. In a private chat, following the link opens a conversation and immediately sends /start <payload>. In a group, the startgroup parameter instead prompts the user to pick a group to add the bot to, and only after that choice does the bot receive /start@your_bot <payload> — one extra step a private-chat link never requires.
Nothing forces the payload to mean anything beyond an opaque string the bot decides how to interpret, which makes it a natural place to encode where a user came from — a specific channel, a specific post, an ad creative — the same job a UTM parameter does for a website link. The difference is delivery: a website reads a UTM tag from the query string on page load, while a bot reads its payload from the very first message the conversation produces, so the attribution arrives before any other data does.
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