K2 and SharePoint allow for embedding an iFrame by adding the Embed App to the Modern Team site. This should work universally for any URL (i.e. SmartForms or Worklist URL).
While trying to embed the site, you may receive this error:
"Embedding content from this website isn't allowed, but your admin can change this setting."
This error occurs because of a SharePoint setting. On the Site Settings page, under Site Collection Administration, click HTML Field Security.
Select the option to Allow contributors to insert iframes from any domain.