> For the complete documentation index, see [llms.txt](https://help.bots.business/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.bots.business/build-bot-behavior/deep-links.md).

# Pass a value when a user starts the bot

Pass a short start parameter into your Telegram bot, read it in BJS, and validate it before using it as a command or referral value.

Use a Telegram start link when a user should arrive with a short value, such as a campaign or product code:

{% code title="Example · Example 1" overflow="wrap" %}

```
https://t.me/YOUR_BOT_USERNAME?start=summer_2026
```

{% endcode %}

Replace `YOUR_BOT_USERNAME` with your bot's username. The `/start` command receives the supplied value as BJS `params`.

## Read a known campaign value

Create `/start`, leave **Wait for answer** off, and use this BJS:

{% code title="Read a known campaign value · Example 2" overflow="wrap" %}

```javascript
if (params === "summer_2026") {
  Bot.sendMessage("Welcome to our summer campaign. Send /help to continue.");
} else {
  Bot.sendMessage("Welcome! Send /help to continue.");
}
```

{% endcode %}

Create `/help` with your next steps. Test a plain `/start`, the campaign link, and a link containing an unknown value. The unknown value should take the normal welcome path.

## Parameter rules

Telegram start parameters support letters, digits, `_`, and `-`, with a maximum of 64 characters. See Telegram's [deep-linking specification](https://core.telegram.org/bots/features#deep-linking).

Treat the value as user-controlled input. It can be edited or reused; receiving a value is not proof of identity, payment, or permission. Keep credentials out of start links and chat responses. Use an explicit allowlist of supported values when the link selects an action.

For referral handling and attribution, use the [referral library guide](/libraries-and-examples/referrals.md) instead of granting a reward merely because a parameter was present.

## If params is empty

Check that the link uses `?start=`, that the user completed Telegram's start action, and that you are reading the value in the `/start` execution. A later command does not automatically inherit the original link parameter. Pass it through [options](/bjs-reference/context.md#pass-structured-options) when calling the next handler; save a property only if a future independent message needs the value.