It depends. If the stock disposition is reported on a tax slip, then it should be available to download using Auto-Fill. You can check in your CRA MyAccount to see if the slip is there. If it is, you can use auto-fill again to add it to your tax return.

If there was no slip issued, then you'd have to enter the stock disposition manually in the Capital Gains section. 

Either way, if you have already NETFILE'd your return and need to add this info, you would need to do a ReFile, as soon as possible to add it. This TurboTax Canada FAQ tells you how you use ReFile: I've filed my tax return. Can I still make changes?