How to Request and Enter Your API Key
Once you have the plugin installed, visit Settings > Postmatic in your WordPress dashboard. Click on the blue button that says Get a free API key.
Create an account with Postmatic.
Enter your site url and click on the blue Add site button.
If you want to go for the two week trial, click on the blue Try Postmatic Premium button. Otherwise, click on the white No thanks button.
If you clicked on the No thanks button, you will be brought to the screen below.
To view your API key, click on Modify, Upgrade, API Key.
If you clicked on the Try Postmatic Premium button, you will be brought to the screen below. Complete the fields in the Payment Details section of the screen. When you're finished, click on the Start my trial button.
Return to your WordPress dashboard and go to Settings > Postmatic. Paste your API key into the Postmatic Key box, then click on the Save Changes button.
Voila! You're good to go.