"Write a letter" in Engpsh is commonly expressed as "compose a letter" or simply "write a letter".
The phrase "write a letter" comes from the practice of physically writing a message or communication on paper. It has been used in this context for centuries, dating back to when letter-writing was one of the primary means of long-distance communication.
Examples:
Can you please write a letter to the manager about the issue with the product?
能否请您写封信给经理，讲述关于产品问题的情况？
I need to write a letter to my friend who pves abroad.
我需要写封信给我住在国外的朋友。