Where in Turbo Tax do I input information about "realized loss on marketable securities" reported to me on a Grantor Letter?

It will be entered in the Investment & Savings section.

Select Wages & Income

1. Click Review/Edit
2. Scroll to Investments and Savings1099-B, 1099-INT, 1099-DIV, Crypto Gain/Loss Report
3. Select the investment you want to post.
4. Select the way you want to Import it, or you can [Type it in myself].
5. Continue the interview.