You cannot buy a Museumkaart abroad without a Dutch address. You can procure a temporary one at participating museums.
You won’t need to provide a (discount) code when booking tickets with a Museumkaart in advance though. You would see Museumkaart option listed (at zero euros cost) when selecting the tickets. You can this already book the tickets in advance for your preferred slots.
However, you will need to be able to present one upon entry. You can procure the temporary Museumkaart at the first museum you visit before having your tickets scanned upon entry. Typically, the entry slot is 30 minutes so you should be able to procure a temporary Museumkaart.