Yes, if your return has been already accepted by the IRS, you will have to file an amended return (1040X) to include the form 1099MISC. The IRS will only accept amended returns via mail.
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
