You don't directly add to the 1040.  You enter your income like W2 and any 1099 you get.  Turbo Tax fills out the actual 1040.  What kind of income do you need to enter?  

You can't see the forms in the Online version.  You can preview the 1040.  You can preview the 1040 or print the whole return https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

You can see and enter into the forms using the Desktop program.
Https://turbotax.intuit.com/personal-taxes/cd-download/