There might be something you could do with CSS to highlight the category selector box so it’s more obvious to people they should start there.
There could probably be some more advanced techniques with JS to not allow people to click the body of the composer until the category box is filled, or to pop an alert that asks if they really want to submit it as uncategorized. Or even an alert that when you click the body box first that nudges someone to choose a category first.
I’m a little unsure what is the solution you’re hoping to see, maybe I just didn’t understand, could you describe it in more detail, ie, the user clicks this, then the user sees this, etc?