AWS Kinesis Data Firehose: Authentication token using a custom HTTP header (or any workaround)

I want to take advantage of AWS Kinesis Data Firehose with a destination of HTTP endpoint to stream text records. The destination expects for authentication an HTTP header of the form:

Authentication: Bearer: xxxxx-xxxxxx-xxxxxx

No other format is allowed, and I cannot do any processing on the receiving side. After doing some testing, I couldn't find a way to have a successful setup. It seems there is no way for me to customize the access key header or to add another header that matches my requirement. This is because KDF sends the headers in the following format. I tried to include the key (10th header) as well as adding additional header (4th header) :

"headers": {
        "content-length": "1286",
        "x-amzn-tls-version": "TLSv1.2",
        "x-forwarded-proto": "https",
#this   "x-amz-firehose-common-attributes": "{\"commonAttributes\":{\"Authorization\":\"Bearer: xxxxxx-xxxxx-xxxxx-xxxxx-xxxxxxx\"}}",
        "x-forwarded-port": "443",
        "x-forwarded-for": "13.100.100.100",
        "x-amzn-tls-cipher-suite": "ECDHE-RSA-AES128-GCM-SHA256",
        "x-amz-firehose-request-id": "xxxxxx-ab4f-4771-a3bf-xxxxxxxx",
        "x-amzn-trace-id": "Root=1-xxxx-5f4c2f236809bxxxxxxxx",
#this   "x-amz-firehose-access-key": "xxxx-xxxx-xxx-xxxx-xxxxxxxxx",
        ....
}

Is there a way to make this work? Thanks