Unfortunately, it is not possible to set up an automatic welcome message for new Instagram followers.
This is prohibited by Meta's company policies. Reacting to a follow action on Instagram is forbidden; you can only check if a user has followed you within an automation.
You can learn how to check for subscriptions in this instruction.
In addition to checking for subscriptions, you can set up welcome buttons for users who haven’t yet had a conversation with you in Direct. Learn how to do this in this instruction.