brokerCommand takes only two parameters. In your case:
brokerCommand (int Command, var* Parameters)
For PLOT_HLINE, it specifically says that the second parameter is var[5].
So in your script, you can do something like this:
var hconfig[5];
hconfig[0] = 0; // always 0
hconfig[1] = 0; // price position
hconfig[2] = 0; // line color
hconfig[3] = 0; // line width
hconfig[4] = 0; // line style
brokerCommand(PLOT_HLINE, hconfig);
