There is no credit or deduction for an assumed loan and therefore no entry point for it in TurboTax.
You will claim your mortgage interest payments as if you were paying interest on a loan in which you originated in the following area:
To enter mortgage interest expenses in TurboTax Online:
Click My Account (Top right of your screen).
In the pop up window, select Topic Search.
In the search bar, type in mortgage interest
See it highlighted, click GO, and follow the prompts