Sunset portraits are always going to be a challenge, owing to the dynamic range required.
I don't have that mode on the D3200, so I don't know if you can manually pop the flash up. I also don't know if there is a possibility here for HDR.
If not, then I'd suggest going to manual mode. Meter on the sky, set your exposure for that, and then use the fill flash. You can alter color saturation in post if the sunset is not intense enough. If you use A or another mode, don't forget to hold the exposure reading, or it will try to expose the subject right and overexpose the sunset.
If you shoot in Raw, you can also use View NX2 or a similar program to get shadow recovery in post and lighten up your subject.
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I don't have that mode on the D3200, so I don't know if you can manually pop the flash up. I also don't know if there is a possibility here for HDR.
If not, then I'd suggest going to manual mode. Meter on the sky, set your exposure for that, and then use the fill flash. You can alter color saturation in post if the sunset is not intense enough. If you use A or another mode, don't forget to hold the exposure reading, or it will try to expose the subject right and overexpose the sunset.
If you shoot in Raw, you can also use View NX2 or a similar program to get shadow recovery in post and lighten up your subject.