Hey Andrew,
Thanks for getting back and showing a screenshot. The key itself should go under Value and the word "key" should go under Parameter. Top of my head I cannot remember if it matters if this is capitalised so I would try this in upper and lower case.
Hopefully that fixes things. I would also suggest deleting that screenshot as the API key could be used by other users viewing the thread. With OpenStreetMap I don't think this could be done maliciously, but it could lead to rate limiting if someone spammed requests to the service.
Hope that helps!
David
Esri UK -Technical Support Analyst