Yes you can , but you'll need to complete a W-8BEN form before you can invest in US shares in any account other than a SIPP. Similarly, you'll need to complete an NR301 form to deal in Canadian shares. These forms allow you to benefit from a lower ‘treaty’ rate of withholding tax.
Both of these forms can be completed from your app. Just go to your settings cog in the top right corner via the 'More' menu and select 'My details' and then 'US & Canadian dealing'.
From here you can complete a W-8BEN form straightaway or go to our website to complete an NR-301 form.