The best place to get 2018 AGI is from the 1040 form you filed with the IRS. Look on page 2, line 7. If you filed a 1040NR, it'll be on line 35.
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
