Log in to your WordPress website's backend, click on Appearance and choose Themes. Find the theme you need to edit, click on three dots and choose Edit. Go to Pages tab and choose the page you want to add the Mailchimp form to. Then click on the template on the right side. In the editor, find where you'd like to show the Mailchimp form banner and click on Add Block. Then find and click Custom HTML. Paste your installation code in the popup window and click Save to save changes.