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Create a Basic Page
Admin menu > Content > Add Content >Basic page
In the basic page settings, below the Body field change the Text Format field to Full HTML
In the Body field select the Source button.
Add an HTML IFRAME with the dashboard URL
E.g.
<p><iframe src="https://arcgis.com/apps/dashboards/abcdefghijklmnpoqrstuvwxyz%22" style="min-height:800px;" width="100%"></iframe></p>
In Menu Settings select the checkbox for Provide a menu link.
In URL path settings set a URL alias (e.g., dashboard-test).
Create a Menu item using the basic page node and the menu link