Contact your bank for a power of attorney form. Authorization letters are formal letters that empower somebody else to take actions on your behalf.How Do I Write a Credit Card Authorization Letter? What are the guidelines to write an authorization letter? Check out this Letter of authority bank sample for your reference. How do I sign up for Direct Deposit? Fill out this authorization agreement along with verification from the bank showing the account number and routing numbers. An Authorization Letter allows you as an account holder to grant another trusted individual to perform banking transactions on your behalf. Bank account information. Month and day of birth.