For Splunk Enterprise, dashboard_html_allow_embeddable_content must be set to true in the appropriate app's web.conf; see this discussion for details. I've tried doing the same thing for a Splunk Cloud instance. Unfortunately, the custom app fails the app validation during installation in Splunk Cloud because of the configuration changes to the web.conf file. I've posted a question about this specifically for Splunk Cloud. I know this post is over a year old but I came across it while Googling for a solution to the problem and thought I'd link to the related posts.
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