Yes, if your return has been already accepted by the IRS, you will have to file an amended return (1040X) to include the W2. But you might want to wait until your refund or payment is processed. The IRS will only accept amended returns via mail.
If your return has been rejected by the IRS, all you will have to do is add the W2 on your return and refile.
The 1040X form for 2018 hasn't been approved yet by IRS. Please, keep checking the following information on how to amend your return; it will be updated once the 2018 form is released:
https://ttlc.intuit.com/replies/3288565