The Realtime application key (aka appkey) is a 6 characters unique identifier of your Realtime subscription.
For unregistered users
To get yours you need to register for a Realtime account at https://accounts.realtime.co/signup/
After you get your validation email click on the email link to validate your registration.
After validation you can login using your credentials at https://accounts.realtime.co/login/
As soon as you're logged in click the option Add subscription.
Select the Realtime Cloud Messaging product, choose your plan (Free, Ignite or Corporate) and click the button "Subscribe" as shown below:
Give your new subscription a name so you can easily identify it on the subscriptions list and click the Create my free account button.
That's it! You'll find your new Realtime application key on the right panel under Application Key:
For already registered users
After you registered and subscribed to a Realtime product, you can access anytime your Realtime application key (and other details) by browsing into your subscriptions list at https://accounts.realtime.co/subscriptions/
Click on the subscription name to expand the details panel and you'll find the application key for that subscription as shown below: