This is an easy one. Write it in raven's feather ink. It is a commanding ink. You burn a black feather (freely given which means found) and burn it. Then you mix the ashes with any black ink. I'm sure you can buy it if you don't have a black feather lying around.
Alternatively you could look into other ink recipes. I would personally do a ritual on the ink itself to charge it for its specific purpose and then use it to write the letter with.