ModBus Write Command

Hello Can you please tell me how to write to a Modbus slave device, this is my configurations
10 VAR1 = 3.1.MB_REG2 = 768

you can write sub device like below
10 3.1.MB_REG2 = 768
3 - > panel of controller
1 → id of sub device

Hello Chelsea and thank you very much for your reply, I have try that one already but no luck, it seems that this registers are also read so with that register like that I was able to read the switch status of the Modbus slave device, Example with an external modbus software I write to this device and I can read on the T3000 that the device change of state so the communication is ok like panel 3 device 1 only the programming to write is not okay,
This modbus device is a 8 Channel switch so If I write 768 to the 1 channel it will be on and If I write again the 768 it will be off,
I want to do this configuration with the VAR 1 over BACnet integration

Your description of the issue isnt very clear, send on some screen shots of what you have tried and maybe we can spot something from that.