Where are taxes withheld from t bill interest recorded in turbo tax?

T-bill interest is reported on form 1099-INT.

Federal tax withheld is reported in box 4 of form 1099-INT.

In TurboTax Online, when you enter your form 1099-INT, on the entry page, put a check mark next to My form has info in other boxes (this is uncommon).

Other boxes will appear and you'll enter Federal tax withheld in Box 4.

on the 1099-int box 4 just like on the 1099-INT you received.