This should be handled via either SysVar or EnvVar.

in Panel, you can assign value to a System variable which can be accessed in CAPL code.

for example,

variables

{

byte xxxxx = 0; //assuming 8 bits data

}

on sysvar sysvar:xxxx // name your system variable

{

xxxxx = @this;

}

on message CAN_0x598
{
    
    if(counterAlive == 15)
    {
        counterAlive = 0;
    }
    else
    {
        counter++;
    }
  
    msg1.byte(0) = this.byte(0);
    msg1.byte(1) = this.byte(1);
    msg1.byte(2) = this.byte(2);
    msg1.byte(3) = xxxxx ; // Changing through the Panel
    msg1.byte(4) = xxxxx +1;  //pay attention on overflow value as it can hold till 255
    msg1.byte(5) = this.byte(5);
    msg1.byte(6) = counter; 
    msg1.byte(7) = this.byte(7);
}