Twilio Flex's CRMContainer uriCallback does not allow http sites

I am going through the Twilio getting started guide here https://www.twilio.com/docs/flex/quickstart/getting-started-plugin and cannot get the UI to display a HTTP site (it works for HTTPS).

flex.CRMContainer.defaultProps.uriCallback = (task) => {
      return task 
        ? `https://bing.com/?q=${task.attributes.name}`  # this works
        : 'http://www.your-site-here.com/api/your-api';  # this doesn't (but works when visited via browser)
    }

Twilio developer evangelist here.

Mixed content is a security risk and could put your Flex instance and user data at risk. Please see this article on MDN about how mixed active content, such as iframes, is dangerous and is blocked by default in browsers.

It is not specifically Flex that is blocking the content, but your browser. You should serve your CRM over HTTPS (or proxy it via HTTPS, if directly serving it over HTTPS is not possible).