If you would like to authorize a joint holder to operate your account, here is a sample template you can use to write a joint account holder authorization letter.
(Bank Name and Address)
Subject: Authority Letter for Joint Account
To Whomsoever It May Concern,
I (Your Name), the undersigned, a loyal customer of your bank since (Year), hold a savings account (Account Number) with you. I would like to authorize (Authorized Name) to operate my bank account on my behalf. (Authorized Name) is entitled to handle all banking withdrawals and deposits under my name. The authorization will be effective from (Start Date) till (End Date).
Thank you for your co-operation.
(Your Name and Signature)