For AI21, you will need to create a Studio account. You can do so by signing up here. Once signed up you will be taken into their own playground. You will want to click the user icon in the top right and click on the Account menu that opens up. Alternatively, click this link directly.
At the top of this page, you will see your email address and your API Key beneath it. You can hit the icon to copy your key to clipboard and that is as simple as things get! Then go back to Riku's API settings and paste it in the AI21 box. You can find that page here.