Its in the proximityprompts properties?
Save the Billboard located under ProximityPrompts (in your PlayerGui) and analyze it. It’s a local file
I looked into the ProximityPrompt script located among the files in the ExtraContent - CoreScripts. The whole UI is hardcoded and starts by using ProximityPrompt.KeyboardKeyCode
and passing it to UserInputService:GetStringFromKeyCode(), which means that most times it’s a letter in a textbox.
If it can’t associate the input with a string character, the script then maps the rest of input with asset IDs listed here (below): UserInputService | Documentation - Roblox Creator Hub.
The white frame is a separate image with the asset id "rbxasset://textures/ui/Controls/key_single.png"
(icon size: 28x30; bigger ones are key_single@2x.png
and key_single@3x.png
).