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 a specific Telegram channel or group 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:
If the IFTTT bot was added to a group, return to the group chat and send the message /start@ifttt the bot will then reply to that message and you can press the /Connect_group option.
Alternatively, if the IFTTT bot was added as an administrator to a channel, first ensure the channel is set to public then go to channel settings and forward the share link to the IFTTTbot.
Once you have completed the above steps you will be able to select that specific Telegram channel or group when you create or connect an Applet back on IFTTT.