how to get cwnd in Wireshark?

If you mean the congestion window, then, to quote RFC 2581, "The congestion window (cwnd) is a sender-side limit on the amount of data the sender can transmit into the network before receiving an acknowledgment (ACK)"; it's a parameter inside the TCP network stack - its value is not transmitted in network packets, so you won't see it as a field in the dissection of a TCP packet.

It might be possible to infer it by seeing the sender sending smaller TCP segments than would be possible given the receive window, but that doesn't guarantee that the number of bytes transmitted is the congestion window value - there might be other reasons (such as not having enough data to transmit to fill up the receiver's window) why the sender didn't send a full window's worth of data.