To use Telegram Applets, you must first add the IFTTT bot to your Telegram account. Here's a video to walk you through the steps of setting this up:
After the bot is added to your account, you will need to make it an administrator for specific Telegram channels that you want it to be active in. Here's a video for how to make the IFTTT bot an admin of a specific Telegram channel or group:
Once the bot is an administrator for a channel, you will be able to select that specific Telegram channel in Applets that you turn on.