Actually, you don't want only to send data, because Function code 03 you are using is used to read output registers of Modbus slave, 32 words starting at register 161. So you will also need to read the response from this slave.
I don't have experience with CP341, but I used ET200S SI module, and it should be similar. You first need to send this you have data using FB3 (S_SEND_SI) - this is request to read. After this FB signal it is DONE, you need to call FB2 (S_RECV_SI) to read the response and place it in some DB.