Where can I find my Modified Adjusted Gross Income in my turbo-tax return

MAGI for what specific purpose? There are different requirements for different purposes.

To determine if I will go into a higher bracket on my Social Security medical premiums.

Turbotax does not calculate that for you, you have to calculate it. It is your adjusted gross income (Line 37, Form 1040) plus your tax-exempt interest income, Line 8b, Form 1040.

See:
https://www.ssa.gov/pubs/EN-05-10536.pdf

For the ACA subsidies.

Please review the accepted solution above @mollys6.

tax free interest is apparently not the only income added to AGI by turbo tax. Do you know which other lines on the return  I add back in the income?

• Start with your Gross Income Line 7b of your Form 1040.
• To get your AGI you then deduct
  • IRA contributions
  • Student Loans
  • Moving expenses
  • One half of self-employment tax (If self-employed)
  • Self Employed health insurance
  • Contributions to SEP, SIMPLE and qualified plans for yourself
  • Any Alimony paid
  • Deduction for Archer Medical Savings Account
  • Penalties paid for early withdrawal of savings
• To get your Modified AGI
  • Any amount excluded from gross income in section 911 (Foreign earned income and housing costs for qualified individuals)
  • Any amount of interest received or accrued by the taxpayer during the taxable year which is exempt from tax
  • Any amount equal to the portion of the taxpayer’s social security benefits (as defined in Section 86 (d)) which is not included in gross income under section 86 for the taxable year.

https://www.irs.gov/publications/p590a