Try with these values in the key: "content_type" in the headers:

"application/vnd.kafka.json.v2+json"
"application/vnd.kafka.avro.v2+json"
"application/json"
"application/octet-stream"

Ideally one of these should make it work

Also, keep the message in this format:

{"records":[{"value":<your message>}]}