Once you've configured a widget and copied the snippet from the widget wizard, installing it means pasting that snippet into the HTML of the page where the widget should appear. The snippet is a single block that includes both the widget's markup and the loader script that renders it.
Don't put the widget in an iframe if you can avoid it. Iframes interfere with the registration flow. If you must use one, enable Open next page in new window on the widget — but note the browser's pop-up blocker may swallow it and the attendee may miss the confirmation.
Generic HTML / custom site
Paste the snippet into the body of your page at the position where the widget should render. For the Card widget specifically, paste the snippet just before the closing </body> tag — this lets the card render before the loader script runs, so it's visible even if JavaScript is disabled.
WordPress
Use a Custom HTML block in the page or post where the widget should appear, and paste the snippet there. For site-wide widgets (Pop-up, Fixed bar), a header/footer injection plugin is simpler than editing the theme. See How do I install eWebinar widgets into WordPress pages? for details.
Webflow
Drop an HTML Embed element on the page where the widget should render and paste the snippet. See Framer or Squarespace for similar builders.
Squarespace / Wix
Use a Code or HTML block. These builders sometimes sandbox third-party scripts — if the widget doesn't render, check the block's script execution setting.
HubSpot
Use a Rich Text or HTML module in the landing-page builder. HubSpot also supports pasting the snippet into the page's source code directly.