you need to cup your hands around the end of the call and create a lot more backpressure then you're used to with an owl call. Any reed owl call will sound like a sick duck if you don't. Get your hands all the way around it, make a big cup like you're closing it off. Then blow, you'll learn how much to ease off with the backpressure your hands are creating until the right sound comes out.
If you want to do the barred owl laugh, then you'll have to almost clap your hand on and off the end to get the result.