In order to redeem this prize, you’ll need to have your Heroes account linked to a Nintendo Account. Furthermore, you’ll also need to connect the same account to your Switch. You can see the full details for redemption here.
The promo announcement doesn’t tell us a whole lot about Byleth, but it does say that he’ll also be available in regular summoning events. It’s looking like we’ll be getting at least one 3H banner in late July.
Heroes will begin promo distribution on or after 26 July. If you can’t get your hands on 3H right away, no worries; the promo will be available until 29 June, 2020.