Struggling to update post using Admin API & Lexical

I’m still trying to understand ?source=html. The challenge I’m facing is that one of my posts that I would like to update using the Admin API has a public preview. The source file uses Markdown.

I see from an earlier thread that you wrote, " ?source=html will pick up the <!--members-only--> comment and convert to a paywall card." Setting public previews via Admin API - #6 by Kevin

Is there a way to create a public preview with the Lexical script that is working for me? If not, what steps do I need to take to be able to create the public preview? Do I have to convert the Markdown to HTML in order to use the members-only comment?