When you enter the interest
Deductions & Credits
Your Home
Mortgage Interest, Points, Refinancing, and Insurance START
Enter the Seller's name
Continue and select "This is a seller-financed loan and I did not receive an 1098"
Continue to enter the information
Since the Seller is not a financial institution, they are not required to issue Form 1098 but they are required to report the interest you paid on their tax return.