You don’t. Webflow takes the HTML or RTF content from the clipboard and normalizes it to the element you’re pasting into. For a rich text element, that’s H1… bold, italics, lists, etc. It does a pretty good job most of the time at normalizing your content to fit that structure.
Style, fonts, color, line height, font size, etc. are all stripped and are dictated by how you’ve styled your rich text element itself. This is exactly what you’d want, to keep your site reasonably consistent.