You can always use the Ctrl-Opt-Cmd- Left or Right arrow keyboard shortcut to change the first line indentation of a paragraph.
Generally, though, you leave this sort of thing to the Compile stage. The logic isn’t actually quite as simple as you suppose, because the user might add a title at the top of the text, and if the user doesn’t use a header level in the title, Scrivener has no way of knowing what the user intends. There’s not even a way of doing this automatically in Word, I believe - even there you have to set a different style for the first paragraph if that’s what you want.
I generally wouldn’t recommend using a style for this sort of thing in Scrivener, though, because that could make overriding format at Compile time just that tiny bit more difficult. I would just decent the paragraph using the keyboard shortcut.
All the best,
Keith