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Hi Saovry sok,

Sorry, but i'm not aware of Khmer input methods, so I don't know how you use keyboard when you write in Khmer on computer.
But I think in AutoCAD you need special font for using Khmer language in annotations:

https://www.youtube.com/watch?v=Et0JfmHk4Rs

https://www.youtube.com/watch?v=J2wwdrf4374