How do I file for a tax extension in 2018 if I haven't finish / submit my 2017 tax return?

You need to estimate your tax liability amount when filing extension. 

Thank you.  What do I put in the AGI question when filing my extension as I haven't submit my 2017 tax return yet?

You can estimate that amount too based on income documents you have for 2017.

Thanks for the fast response.  I input my estimate amount and submit the extension.  IRS rejected my extension application.

What is the reject code?   it should be in the rejection email.

Try entering zero for last year AGI

I tried to reply with the reject code but it's not allowed.  I will submit extension again with AGI $0.  Thanks