OmGv,
There's no standard letter to send expedite request.
I have tried 3 times, however it did not work in my case. The third time BCIS replied back that if i try again, i won't get a reply back from them as my expedite request has been denied three times. Also, i was told that first i need to file Advance Parole and then only i can request expedite if i am planning to leave the country. (i have vaild H-B, no ead, no AP).
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Believe me if you don't have a good case with support papers to sumbit your case won't be considered.
However, for Advance Parole, you can try medical grounds if someone is sick in your family.
I hope it helps.