The data entry depends on the nature of the loan:
If this is
a personal loan, the interest is not tax deductible, unfortunately.
If this a business loan (I'd like to add this option here, as you are typing from TurboTax Self-employed), you may deduct the interest as your business expense; simply revisit Business expenses section, you may put your own description and amount for the deduction.
If this a personal loan that meets the requirement of a secured debt (such as mortgage), you would deduct the interest as a regular mortgage interest.