Using AysncUDPMessage for ESP32 to send string data
Use udp.print(row.postString().c_str())

AsyncUDPMessage in AsyncUDP library is only a buffer. It is used only in AsyncUDP send function:

size_t AsyncUDPPacket::send(AsyncUDPMessage &amp;message)
{
    return write(message.data(), message.length());
}