Is there any way to set it up so that Scrivener can mimic the screenplay scene heading auto-detection that FadeIn (and presumably Final Draft) have? In FadeIn (another screenwriting software) whenever you’re in an Action line and you type INT or EXT it auto-detects that you want to switch to a scene heading and it does that for you, instead of having to semi-manually initiate it in Scrivener.
This difference in functionality seems minor but the difference it makes to the flow of writing a screenplay has been noticeable to me and has had me back and forth between which of the two softwares to write the current draft of my feature in.
You might try using Keyboard Maestro to create macros to speed your writing flow.
For example, I have a macro that fires from control-I which makes a new scene heading, types INT. and hits tab. So with a single keystroke, I get the slugline and the INT. ready to go. I have another on control-N that does a tab, - NIGHT and a Return.
This kind of thing really improves the flow of your writing.
I’m also struggling with this exact feature. I’m finding plenty of information about how to add strings of text to autocomplete when the element is already established, but the elegance of FadeIn is that simply typing int. or ext. , even if you get the capitalization wrong, it will convert the element to a scene heading without any extra steps. If anyone knows how this can be achieved natively in Scrivener please let me know.
AFAIK, you can’t achieve this natively in Scrivener. If you look at the Completion List in Screenplay Settings, you’ll see that there is no way to use an auto-complete to change the element type.
I do heartily recommend Keyboard Maestro to solve this problem. Look at how easy it is:

I built this in five minutes with four steps in the macro builder:

I find Keyboard Maestro to be a strong compliment to Scrivener.
They offer a free trial:
keyboardmaestro.com/main/