See the following FAQ and video:
FAQ: How do I find last year's AGI?
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947
If you used Online TurboTax last year, log in, and at the Tax Home scroll down to "Your Tax Returns & Documents." Expand that section and choose tax year 2018. Then look for a link "Download/print the PDF" or "View adjusted gross income (AGI.)"
If used online TurboTax for 2018, but are unable to find the 2018 online return in the same account you're using for 2019, we can tell you how to look for/access multiple accounts.