You must be on a Business plan on WordPress.com in order to install the Zapier plugin. The plugin in WordPress.org is free of cost to install.
Before you get started creating a Zap with WordPress, you’ll need to install the Zapier WordPress plugin.
When you start creating a WordPress Zap, you will be asked to connect your WordPress account.
Next, you’ll be asked to enter your WordPress URL, Username and Password. The Base URL will be the full publically accessible WordPress URL e.g. http://wordpress.examples.com/
More details are available on the plugin Installation instructions, check the below link:
Connecting WordPress to Zapier if you wish not to create a user account. To create a new user, follow these steps:
Paste your key back into your Zapier account and then Continue.
If all steps were successful your WordPress account will now be successfully connected.