After clicking Start, you will see a message indicating the API key and Secret key. You will need to add this information on the Telegram page on Tilda.
Go to Site Settings > Forms > Telegram
Add the API key and Secret key that you've received from Telegram to the form. If you have your own domain or you plan to have one in the future, go to General Form Settings where you should specify all the domain options where you plan to place the forms.
If you did everything correctly, then Telegram will receive the message: "Data sources Telegram added on Tilda", and the page with the list of receivers will be uploaded.
Return to the page where the form is located and open the menu of the Content block. Tick the Telegram checkbox. Then publish the page. The form is ready.
If something isn't working, make sure that your page opens at the same address as the one you've specified in form settings.