TurboTax should have generated Form 8453 when you selected that you want to enter a "summary" of your Coinbase transactions.
You aren't able to attach the Coinbase detail information when you E-file however, per the instructions for Form 8453, you have three days to mail in your Coinbase information as support for your "summary" along with Form 8453, after the IRS has accepted your return. There's no cover letter.
Mail Form 8453, with your attachments to:
Internal Revenue Service
Attn: Shipping and Receiving, 0254
Receipt and Control Branch
Austin, TX 73344-0254
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