Displaying a Value while linking a Key with the Linked Field Module
You need to configure the "token" view mode for the content type in question.  

First, ensure you're set up to configure the "tokens" view mode separately from your other view modes.


Click to go to the "default" view mode:

Near the bottom, click "Custom display settings" and check "tokens", then save.

Now you will see "Tokens" in your list of view modes to configure:



Go to that page, find the field that contains the key you want and, in the drop-down in the format column, change it to "key".