If you received the offer in an email, then you should be able to use it even if you live in Canada. Make sure you are logged in to your account on the main Sonos website.

Yes, I received the offer to my email, and yes I’m logged in. At checkout the dropdown menu only allows “US” option when selecting the country for delivery. No other options are visible.

In the UK but if you go down to the very bottom of the initial web page, change the region to where you are, then login (if needed) and look at your offers section in the account tab, you should see the 20th anniversary link. 
 

David

Awesome suggestion. This worked! Thank you ​@Dhouston 

Very disappointing to be clicking on the 25% off on 20th avviversary link in the email and realise it appears to be an offer for only customers in the USA. 
here’s an idea Sonos, just email your customers who are registered in the USA, not every Country. 

Even the new email gives problems.  It still sends you to the US site, even if already logged in. It still lists prices in dollars.  You can then change to UK for example and it still shows full prices and even asks you to log in, even though you can see your profile.  You can add things to your basket and they say they're added but then says the basket's empty.  Then, randomly navigating a few pages will show prices with the discount.  Really inconsistent.

Thanks. Got it sorted. Like you said go to the very bottom of the page and you'll see the flag of the current page. Just click in and go to your country.

It could have been made more clearer by putting it further up where you can see it or even right at the top where you can't miss it.