public void SendMessage(string destination, string symbol, params object[] args)

where the destination is the receiver box, and args is supposed to be the array of values. I don't know what will be used as a "symbol" in the message box.

The method is triggering the message box but can't change values.

Can anyone please tell me how to change values in the message box from unity, or what will be used as a "symbol" in SendMessage() method?

In the documentation, the symbol is defined as "The message keyword".

I've attached a PD patch below for reference.

Attach a [print] to [r unity_message] and screenshot the print in the Pd console and upload it.
A more useful [print] is [rawprint] which is in the zexy library.... that gives more information.

David.

Here's is the code I'm using:

public LibPdInstance pdPatch;
public float[] list = {880, 440, 440 };

void OnTriggerEnter(Collider other)
{
    if (other.CompareTag("Player"))
    {

        pdPatch.SendMessage("loadbang", "list", list);
        pdPatch.SendBang("unity_bang");                        

    }
}

I'm using loadbang as a destination, and testing using values {220, 220, 440} , {880, 440, 440} and {220, 440, 880}

I suggest you try (as you cannot see the patch or the Pd console) debugging using the libPd "Pipe Print to Console" feature......... https://github.com/LibPdIntegration/LibPdIntegration.
With [print] objects in your Pd patch, connected to the [receive] objects, you can then see in the Unity console any messages arriving in the patch.
David.