Authorization letter should be issued by the Account holder that he is allowing you to use his/her Bank account to get money in and there should be mentioned that you can fully control the account.
This is only applicable, if you are trying to transfer money to a Bank account that is not under your name. Why not just open a Bank account under your name, its easy in Bangladesh. And most importantly you can use this in future for saving and on many purpose. It's really good to have one under own name.