Wait until you have received your 2018 Notice. Then file a T1-Adj. for 2018, for the item in question. When you send in the T1-Adj. be sure to attach a copy of the T5 plus sign and date the form.
As an alternate, if you have My Account, you can use the Change My Return feature.
After filing the adjustment, you should receive a Notice of Reassessment from CRA.