Investment (margin) interest deduction is claimed on Form 4952 Investment Interest Expense Deduction and the allowable deduction will flow to Schedule A (Form 1040) Itemized Deductions, Line 9 to be claimed as an itemized deduction, up to the amount of your investment income.
To enter the information for Form 4952 in the TaxAct program:
Tax Reform Update: Most of the miscellaneous itemized deductions you may have claimed in the past, including investment expenses, have been suspended under the new law. These deductions include:
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.