Investment (margin) interest deduction is claimed on Form 4952 and the allowable deduction will flow to Schedule A, Line 14 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: