Typically for tax purposes until one disposes of an asset (sale/transfer/trade) there is no loss to account for on tax returns. So in bankrupt situations you would have to enter a sale of an asset in the "Investment" topic. Your basis will be what you paid for the asset. The proceeds may be zero or some small number if the company bought the asset (stocks for example) back for pennies. You will indicate the proper 8949 category, for example C or F since no 1099B will exist/be issued. The sale or exchange date will be the last day of the tax year.
You will need to find/keep proof of the asset becoming worthless and/or show some assurance that the asset will not/can not return to value, as well as documentation of when it became worthless. "Losses (Homes, Stocks, Other Property)."
Proof showing the company no longer exists and/or the asset is no longer traded should be acceptable.