Unfortunately, once a return has been accepted by the IRS, you cannot add a W2 or 1099 without amending the return.

TurboTax can help you through the process of amending your return. The amended form is 1040X. As of right now, the IRS has not released the 1040X for tax year 2018. Please check back in the next 1-2 weeks to see if the form is released. 

The process for filing your amended return with TurboTax will be much the same as the 2017 instructions. Here's a link to those instructions.

https://ttlc.intuit.com/replies/6602873