#1 Launch the Bot π
Go to the bot that the developer sent you.
Press /start or tap βStartβ.
Result:

After that, proceed to step 2.
#2 Find Your Telegram User ID π
Go to the bot @userinfobot and press /start.

In my case, my Telegram User ID is 1494778489.
Add your ID to the admin panel / send it to the developer / etc.