How do I add a URL rewrite for request headers

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IIS

httpPlatformHandler doesn't pass REMOTE_USER as a request header. I was hoping to use URL Rewrite but the interface is being difficult. I keep getting a message that there are data errors but it doesn't tell me what is wrong.

What I have tried:

I installed the latest URL Rewrite. I added 'X-Remote-User' as an allowed variable. Adding a rule to assign REMOTE_USER to it fails to save. I don't need to rewrite the URL like all examples have. I only want to save the REMOTE_USER server variable to the X-Remote-User request header.