There is no "assign". In principle, you can translate one into another using one more input source: some mapping rules between those different formats. You can get some ideas on how to do that if you see the source code in this two CodeProject articles:
WPF HTML Supported TextBlock
A Professional HTML Renderer You Will Use
They won't solve your problem, just will give you some ideas — the projects use Rich Text to represent (render) HTML. As to your problem, you should at least formulate it. "Assign" HTML to something is closer to importing of HTML, but you mention "save as HTML". Those two concepts are almost the opposites. First decide what you really want to achieve. And, first of all, always explain your ultimate goal when asking questions.