TurboTax does not support tax losses from investment fraud. We recommend consulting a tax professional if you've experienced this situation.
IRS Form 4684 is available using TurboTax but is not advisable for you to complete the form for a Ponzi type loss without professional assistance.
Using the TurboTax desktop editions -
Click on Federal Taxes (Personal using Home & Business)
Click on Deductions & Credits
Click on I'll choose what I work on
Scroll down to Other Deductions and Credit
On Casualties and Thefts, click on the start button.
IRS From 4684, Casualties and Thefts Instructions page 7 Losses From Ponzi-Type Investment Schemes - https://www.irs.gov/pub/irs-pdf/i4684.pdf#page=7