App Registration: CustomKeyIdentifier is NULL

Hi

I'm comparing the manifests for two App Registrations in AAD. One of them works* and one doesn't. 

Looking at them side by side, I can see two passwordCredentials entries with values like this (in the working one)

"customKeyIdentifier": "***************************" (the value is actually a long string of characters)

In the one that isn't working (I just get "Unauthorized" every time I call it) the values are null:

"customKeyIdentifier": null

This seems like it might be the cause of my problems so I was wondering what I'm doing wrong? How can I get a sensible value in there through the Azure UI?

Thanks

* i.e. our logic apps can use it to call a custom API (using JWT/OAuth) hosted on-premise)