Basically, this is simply a short-term capital loss that is reported on Form 8949.
See https://turbotax.intuit.com/tax-tips/irs-tax-return/how-to-report-non-business-bad-debt-on-a-tax-ret...
However, since the loss occurred in 2018 and was not used in that tax year, you just need to carry it forward as a short-term capital loss. TurboTax should do that for you if the 2018 return was transferred in, but you can also enter capital loss carryovers in the Investment Income section of the program.
