How do I get the API key?

tl;dr: you sign up and obtain it from My Profile page.

Here is a step-by-step instruction:

  1. Sign up to the portal here.
  2. Sign in using the credentials you provided in the previous step here.
  3. Navigate to My Profile in the main menu.
  4. Click the Show link either next to the Primary or the Secondary key prompts. This is your key! In your actual application, you have to provide it as Ocp-Apim-Subscription-Key header.