Hi Mark. The Populate Anything plugin from Gravity Wiz can do that with the “Live Merge Tags” features:
Live Merge Tags.
Add auto-updating merge tags anywhere inside your form (e.g. labels, descriptions, choices, values, HTML content).
Without a plugin like that, you would need to create your own JavaScript to copy the value from the dropdown to another field in the form once a selection is made.
Thanks, Mark. Once you have it, I guarantee you’re going to find a million uses for it! Their support is great, and they stop in here occasionally in the event you need direction that’s not quite a support ticket.