Follow these required steps to get your 5-digit USSD PIN for First Bank transfer:

1. Dial the code *894*0# on your Firstbank registered mobile phone. Mtn, Glo, Airtel or 9 Mobile.
2. Choose from the list of ATM debit cards linked to your account
3. Enter the 4-digit PIN of the ATM Debit card.
4. Then, create a new 5-digit PIN
5. Confirm your 5-digit PIN
6. Receive a successful transaction SMS on your phone.

You can read this post on Firstbank transfer USSD code and other useful codes