How do I specify in TT that my NOL carryforward was from 2017

When you are signed into TurboTax you can enter your 2017 carryover and then use the explanation box to clearly state it is from 2017.  TurboTax knows the required limits.

Keep the letter in your files as an ongoing record keeping document, you do not need to send it to the IRS.  They want it only upon request and most likely are not really looking at it.

• Most taxpayers no longer have the option to carryback a net operating loss (NOL). For most taxpayers, NOLs arising in tax years ending after 2020 can only be carried forward. The 2-year carryback rule in effect before 2018, generally, does not apply to NOLs arising in tax years ending after December 31, 2017. The CARES Act provided for a special 5-year carryback for taxable years beginning in 2018, 2019 and 2020. Exceptions apply to certain farming losses and NOLs of insurance companies other than a life insurance company. Also, for losses arising in taxable years beginning after Dec. 31, 2020, the net operating loss deduction is limited to 80% of the excess (if any) of taxable  income (determined without regard to the deduction, QBID, and Section 250 deduction over the total NOLD from NOLs arising in taxable years beginning before January 1, 2018.
  • IRS Publication 536 (IRS still has in draft)

Here's how to enter a 2017NOL carryforward in TurboTax:

1. Do a Search (upper right) and enter "nol" 
2. Click on the "Jump to....." link
3. Enter your NOL on the screen that appears