23 December 2019
There is no specific format for the same. It may be drafted very easily either in the form of letter or in the form of written submission is support of additional evidence admission. You have to mention the documents which are produced as additional evidence and also specify clearly the reasons why same could not be presented before the AO and why the same should be admitted as an additional evidence as per Rule 36A.
You may also cite a few relevant case laws in support.