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What I understand there’s no formal document. But you need to make something like an excel file which shows the cost basis of your bitcoin, and how you plan on taxing it in the future (I.e FIFO or HIFO). And then you time stamp it with something like https://opentimestamps.org
You don’t need to submit anything, but if the IRS comes knocking in the future you have this document with a time stamp.
This was a good video on the subject.
Good luck. I’ll be doing this Monday.