Dear Prof.XXX How are you? Please excuse me for taking your time! Introduce your self. Thanks for offering the opportunity for reviewing manuscripts for XXXX. I am preparing to apply for US permanent residence. My lawyer told me that judging others' work will give me some credits for the application. I am wondering if you can write a letter to show that I have been serving as a reviewer for the journal " XXXXXX". I understand you are very busy. So I will write the first draft for your convenience, which will save your time. I appreciate your help very much.