On the screen where it asks for your modified adjusted gross income (MAGI), it tells you what TurboTax (TT) has calculated as your MAGI. Most people enter that number in the box. TT provides a blank box in case you qualify for additional modifications.*
Otherwise, you don't "find it", you calculate it.
*Click on Learn More, which says, in part:
Modified Adjusted Gross Income
Modified Adjusted Gross Income is your Ohio adjusted gross income (Ohio IT 1040, line 3) plus your business income deduction (Ohio Schedule of Adjustments, line 12). If you did not take a business income deduction, your modified adjusted gross income matches your Ohio adjusted gross income.
Additionally, if you live in an earned income tax base school district, your taxable income is limited to only income included in your Modified Adjusted Gross Income.