Before you can add Google reCAPTCHA to Form Blocks, Newsletter Blocks, promotional pop-ups, and product waitlists, enable it in Advanced Settings:
In the site Settings panel, click Advanced, then click External API Keys and scroll down to Google reCAPTCHA.
Click the link to get your Google reCAPTCHA API Keys. The link will open in a new tab.
Click Admin Console.
Enter a label to help you identify your site. This can be your website URL, or a nickname.
Click reCAPTCHA V2. Squarespace doesn't support reCAPTCHA V3.
Click "I'm not a robot" Checkbox. The other options aren't supported.
Enter your built-in domain and click the + icon. Use the format yoursite.squarespace.com, excluding https:// from the beginning.
Enter your custom domain and click the + icon. Repeat this step for any other custom domains connected to your site, excluding https:// from the beginning of each one.
Ensure you've added all domains associated with your site. If you don't, you'll see an error.