It's not clear whether you are entering the interest in the Step-by-Step interview or in forms mode.
In the interview, enter the interest as if you received a Form 1099-INT, even if you didn't actually get one. All you have to enter is the payer's name and the total amount of interest. After you enter the 1099-INT, the next screen says "Tell us if any of these uncommon situations apply." Check the box that says "I need to adjust the taxable amount" and click Continue. On the next screen, enter the amount that you sent to the trust. For the reason, select "I received all or part of this interest for someone else (I am a nominee)." TurboTax will make the appropriate entries on Schedule B to show the subtraction.
If you are using forms mode, do not enter the interest directly on Schedule B. As with the interview, you have to use a Form 1099-INT, even if you didn't actually get a 1099-INT. On the Form 1099-INT Worksheet, enter the payer's name, and enter the total amount of interest in Box 1. Then scroll down to the Adjustments to Interest section near the bottom. Check box N for Nominee interest, and enter the amount that you sent to the trust as the adjustment amount, as a positive number. TurboTax will then make the appropriate entries on Schedule B to show the subtraction.
