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I had found a sample letter on this portal. Just search for the key word "sample letter" or something like that. i am sure you will find that. Even if you cannot find a sample letter, you can make one on your own, just like any other business or formal letter.
Just make sure that it contains your case number , receipt date,notice date and other important info regarding your case and in the second para a detailed description of your problem.
I think they can take it from there.
All the best.