Pasting HTML into a WYSIWYG Editor Without Losing Formatting

[Perfecto]

I need to paste HTML into a WYSIWYG editor, but when I paste it, it is interpreted as plain code without any formatting. I've tried using BBCode and Markdown, but these didn't work either. So far, the only solution I've found is to use: https://codebeautify.org/htmlviewer. Do you have a JavaScript or C# script that can do this?