You cannot do that with online TurboTax. Online does not have direct access to forms. Forms mode is a feature of desktop (CD/download) software that is not available for online TT. You can only peek at your Form 1040 before you file.
PREVIEW 1040
https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing
Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.