How do I obtain my 2018 tax return in order to find my AGI? I have tried contacting the IRS but could not be verified because, I don't have the necessary credentials.

If you used the TurboTax online product to file your 2018 tax return, you should be able to access the AGI using this link: https://ttlc.intuit.com/questions/1900840-how-do-i-find-last-year-s-agi

If you can't e-File your other option is to file on paper via US Mail, like in the old days.