I think theres a spot for it on the 1099DIV additional worksheet when you double click on a dividend entry line....or maybe its the 1099INT additional worksheet.

It goes on the 4952 for investment interest.

After that form determines if it is deductible or not, it Should be automatically transferred to schedule A.

Margin interest that your client paid, is an investment expense. If your clients itemizes, it will be put on 1040 schedule A