You will need to wait till you receive your Notice of Assessment- Its is very possible that CRA had it already and if so they would of added it for you. If you notice that it wasn't added then do the following
I have the same issue here, receiving a T4A in the mail after I'd already netfiled. If the CRA were to catch the mismatch, would I be penalised?
@MagicBoo As Brenda said above, it’s likely that CRA will see the error and add the missing slip for you. You’ll see if they did when you get your Notice of Assessment (NOA). If they don’t add it, then you can use ReFile or CRA’s My Account to make changes to your return, as this FAQ says: https://turbotax.community.intuit.ca/community/tax-return-amendment/help/i-have-filed-my-tax-return-can-i-still-make-changes/00/873608
It’s unlikely you’ll be penalized. Even if you have to pay additional tax because of your T4A, the CRA doesn’t start charging interest until May 1st, so you have plenty of time to get this sorted out.