If you would like to authorize someone to operate your account, here is a sample template you can do to write an account authorization letter
Subject: Account Authorization
I, the undersigned [Name], residing at [Address], is a holder of a bank account having account number [Mention the number] in your bank [Bank Name].
I hereby authorize [Name] to handle and assess all the functions related to this bank account. This authorization letter is issued because [Mention the reason]. I hope you will consider this and allow [Name] to manage my account.
I am enclosing the bank account authorization form and my identity proof for further process.
In case you have any doubt regarding this authorization letter, kindly reach me at [Email/contact number]