Dear (insert contact name here, or use ‘Sir’),
Paragraph 1: Because these letters are normally quite short, it is acceptable to get into the main body of what you are writing about straight away. There is no need for introductions. Explain that you are in a position to allow the authorization of whatever is being requested. You should state in this paragraph the exact name of who is being authorized, and what they are being authorized for. Be sure that you have covered all that you need to and make sure you are not allowing more authorization than you initially intended.
Paragraph 2: A short sentence explaining when this authorization will be valid until and you should also ask for them to get back in touch with you at their earliest convenience in order to confirm receipt of this letter.
Name and Position title