Hey SandwichGuy,
You can manage your subscriptions from the Meraki Go portal. If this is your first subscription, you'll be prompted to provide it when creating your Meraki Go account (from the portal). If this is an additional subscription, you can add it by logging into the portal, and then clicking "add subscription" (if you don't see "add subscription", click your name in the top right, click manage products, and then click add subscription).
More information about subscription keys can be found in this KB doc.