Using an output binding for Eventhub it was possible to add furhter message properties like Key/Headers in the v3 programming model, as shown in the examples:

context.bindings.outputKafkaMessage = ["{ "Offset":364,"Partition":0,"Topic":"kafkaeventhubtest1","Timestamp":"2022-04-09T03:20:06.591Z", "Value": "one", "Headers": [{ "Key": "test", "Value": "javascript" }] }",
    "{ "Offset":364,"Partition":0,"Topic":"kafkaeventhubtest1","Timestamp":"2022-04-09T03:20:06.591Z", "Value": "two", "Headers": [{ "Key": "test", "Value": "javascript" }] }"]

This also worked for the Eventhub bindings. But it fails in the v4 programming model, as the whole object is just produced as a message to Eventhub.

Migrated my existing Azure function to v4, set the specific output.eventHub property and testes producing messages to this binding.

Instead of receiving the Value property as a message with the Key and Headers properties set as Key and Headers respectively the whole object was published as a message to Eventhub, with no set Key or Headers.