Here are the steps as outlined in the Quickstart Guide
https://codex.www.ads-software.com/WordPress_Quick_Start_Guide
“Here’s where it gets a little bizarre. You have to sign up for a WordPress.com account before you can get a WordPress.com API key, even though your blog has nothing else to do with WordPress.com.
To get an API key:
1. Open a new browser tab and go to https://wordpress.com.
2. Click the Sign Up Now button.
3. Complete the fields.
4. In the option button at the bottom, select “Just a Usernameplease”,rather than “Gimme a Blog.”
5. Click the Next button.
6. Verify the email WordPress.com sends by confirming it in your email inbox.
7. Log in to your WordPress.com account by entering the username and password you set up.
8. From the My Account drop-down in the upper-left corner, select Edit Profile.
9. Copy the API key that appears right below the header in the intro text. It’s a number like 69368a463006.
10. Return to the admin panel of your blog.
11. Click the enter your WordPress.com API key link that appears in yellow highlighting near the top of the page.
12. Paste the key in the field, and then click Update Options. If the key is accepted, the prompt to enter your API key disappears and plugin is activated