Thank you for using our services. By default the ContentEditor will strip form and script tags when pasting HTML inside. One option would be to map an external template for the HtmlField template used in the ContentBlock widget designer, and in the custom template specify this.editControl.ContentFilters = Telerik.Web.UI.EditorFilters.None; For your convenience I'm attaching a sample modified template. For more information on mapping external templates to Sitefinity 4.x widgets, please check these resources:
Customizing Sitefinity 4 Controls with the ViewMap
Mapping External Templates for Sitefinity 4 Widgets
If any issues persist, can you please let us know of the exact use case scenario that you need to implement, so maybe we could provide you with a possible workaround or give you a more specific advice.
All the best,
the Telerik team
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