In your Mailchimp account, go to Lists => Signup Forms => Embedded Forms.



Choose the Horizontal embedded signup form design option.



Copy the underlined code snippets.



Go to your WP admin panel => Customization => Subscribe Form.
Paste the first 'action' snippet after the 'form' word.
Add the style snippet above the Subscribe button part of the code.



You may remove the optional part of the code. Your code might look like this: