Hi, I applied my 2018 refund to my 2019 taxes. Is that captured under estimated tax? Or, somewhere else?

It is considered an estimated tax payment and is included in that section. 

If it didn't transfer over from last year then enter a refund from last year that you applied to this year here. BUT first make sure it's not already there. You don't want to apply it twice.

Federal Taxes
Deductions and Credits
Then scroll way down to Estimates and Other Taxes Paid
Other Income Taxes - Click the Start or Update button

Next page scroll down to 2018 Refund Applied to 2019
Click Start or Update by the tax you applied

A refund applied from last year will be on 1040 Schedule 3 line 8 which flows to 1040 line 18d.