Where do you enter the Payer's TIN found on form 1099-INT?

If you are in the desktop, it is not required.   It is entered when you enter the form in TurboTax Online.   It goes in the FEIN field.  

At the top of Turbotax (if using the desktop version), click on "View", then "Forms". Locate your 1099-INT on the left side and click to highlight it.  On the form in the right pane, scroll down to the bottom of the 1099-INT and you will find it under the section "Additional Payer and Recipient Information".