Iframe in dashboard

You need a dashboard widget which contains the iframe HTML. Review the source code for existing widgets at /wp-admin/includes/dashboard.php. Your site may need to send a CSP “frame-src” header to allow embedding in another site’s iframe. See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-src

iframes have become more problematic since CSP started being enforced.