There's two type of notification in Midtrans, Transaction notification and Pay Account notification. Transaction notification is sent when a customer completes the transaction or when the transaction status changes. While Pay Account notification is sent when user successfully linked account or when the account is unlinked.
Merchant's notification URL can be configured on merchant dashboard, payment notification URL for Transaction notification endpoint and pay account notification URL for Pay Account notification endpoint.
Ensure that email notification in General Settings is set to ensure various operations work as expected (e.g. save card / one click payments).