To amend your Federal return the form is 1040x & your original return must be accepted. To add your missing W-2 you should wait until the IRS fully processes your return, the IRS gets copies of all W-2's so they may send you a letter asking for the additional information.
If your original return gets fully process without the missing W-2 you would then amend your return to add the W-2. The amendment form is not available to sometime in mid February, you need to wait until the form is avail to amend your return. Amend returns must be mailed & any refund due with the amended return is sent by check.
If you used TurboTax Online, simply log in to your account and select “Amend a return that was filed and accepted.” If you used our CD/download product, sign back into your return and select “Amend a filed return.” Amended returns can take up to three months for the IRS to process.
https://turbotax.intuit.com/tax-tips/amend-return/10-tax-tips-for-filing-an-amended-return/L2Tt29kwj
Your amended return will take up to 3 weeks after you mailed it to show up on the IRS system. You then can check the status of your amended return
https://www.irs.gov/filing/wheres-my-amended-return
