Get specific header value in webapi

I call an Appian WebApi from an external application. It's a webapi to upload a document. Upload work fine but I want to start a process and an parameter of my process is a value stored into the header of the request. IO can't retrieve this value. To test I return it in httpresponse.

a!httpResponse(
       statusCode: 200,
       headers: {
            a!httpHeader(
                   name: "Content-Type",
                   value: "application/json"
            )
       },
       body: a!toJson({
                     "id": document( http!request.body,  "id" ),
                     "originalId": index(http!request.headers,"AppianDocumentId",0),
                     "user": http!request.headers.user
       })
),

OriginalId and user are always null. 

If I use the TestRequest, value are present.

Also If I just return request.headers, values are well returned.