Nowhere really. There is no single MAGI number - the "M" is Modified AGI and depending on which section of the tax law you are asking about,it can be modified in various ways. Each section of the tax code that uses MAGI specifies how it is modified for that section ONLY and does not apply to other sections that define it differently.
Exactly winch MAGI are you asking about?
You can enter the Roth contribution this way and TurboTax will tell you if it exceeds the allowable limit. You can also print all forms and worksheet and view the "Roth IRA Contribution Limit Worksheet" line 1 that is your MAGI for Roth.
Or refer to IRS Pub 590A worksheet 2-1 for the MAGI calculations:
Enter IRA contributions here:
Deductions & Credits,
I’ll choose what I work on (if that screen comes up),
Retirement & Investments,
Traditional & Roth IRA contribution.
OR Use the "Tools" menu (if online version under My Account) and then "Search Topics" for "ira contributions" which will take you to the same place.
A Roth IRA contribution does not actually go on a tax return, but you should enter it anyway to:
1) Tell you if your income qualifies you for a contribution and warn you if it does not.
2) Check if your income exceeds the limit to contribute to a Roth.
3) Track your contribution year-to-year if you use TurboTax every year.
4) Add the Retirement Savers Credit if you qualify.