I renamed your title so it’s clear what you’re requesting.
And here is a screenshot from Typora. They do allow indenting the first line by using tab. And by default it isn’t indented. And then if you export - to PDF for example - that indentation remains intact.
There are likely some CSS wizards who know how to add a snippet that could handle this. For example here is a thread where someone was trying: Indenting paragraphs (and paragraphs only) in source mode