Even they didn’t say on website that you have to stay at the country that you want to apply but they want entry stamp for sure. That’s why you have to go the country you want to apply
For honestly I suggest you not to apply at Cambodia.They take too time and I have no idea why they take too much time even they don’t need to request more documents
I suggest you to apply at Vietnam.It super fast and get approved within 48hours if your documents are perfect