There is no place to record that. If you sent an extension request by mail then the IRS knows you have an extension. If no tax was due, it is a moot point anyway because there is no penalty for filing late if you do not owe or are getting a refund. There is no place to enter on your tax return that you have an extension. You will be able to e-file up until October 15, and then e-filing will close down.
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