Yes, you would need to amend your 2018 return to add the additional W2.  You cannot amend your 2018 with the Online version.  See the following on how to amend your 2018 return.

How do I amend a 2018 return in TurboTax?

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579