First I would recommend creating a snapshot in the Metadata Template with only that field populated. Then, you could first go through the images and apply a color label (example: green for headshot, blue for landscape and so forth). Then, from the edit menu, select images by their color class (or just use the corresponding keyboard shortcut, shown in the menu) and then load the corresponding snapshot in the Metadata Template and apply it to your selection.
If you use a lot of IPTC Scene codes, this process could become a bit onerous/repetitive, so if you are interested, the process could be automated a bit further by utilizing variables and code replacements. At that point you'd only need to apply the Metadata Template once to all images with a color label and it would populate the IPTC Scene field for you. If you are interested in learning more about that, let me know and I'll probably make a video recording to show you how that would work.
