423b is a US designation for ESPPs that encourages longer term holding for preferred long-term US capital gains rates, so if you're dual-reporting, it's likely going to be one of those wacky situations where you pay capital gains tax in Canada for your CRA return, and then also have to pay additional top-up income tax in the US on your IRS return.
If you're only reporting in Canada, you'll be subject to Canadian tax law and this likely will only incur capital gains taxation as we don't have separate short- and long-term capital gains rates.
IANAA, so if this is a meaningful dollar amount you'll want to consult a qualified tax accountant first.