I have a client written for some time ago that uses the old library and does call GetBody to read the body when receiving messages.
Now
The scenario is described here. You will need to serialize the message following this approach:
var serializer = DataContractBinarySerializer.Instance;
using (MemoryStream stream = new MemoryStream())
{
serializer.WriteObject(stream, some_string);
var msg = new Message(stream.ToArray());
var client = new Microsoft.Azure.ServiceBus.QueueClient(ConnectionString, Queue);
await client.SendAsync(msg);
await client.CloseAsync();
}