Look into the manual for more information:

VECTOR object_pos;

PANEL* pnl_show_xyz = {
digits(20,20,5,*,1,object_pos.x);
digits(20,40,5,*,1,object_pos.y);
digits(20,60,5,*,1,object_pos.z);
flags = SHOW;
layer = 1;
}

function update_position_startup() {
while(!player) { wait(1); }
while(player) {
vec_set(object_pos,player.x);
wait(1);
}
}

should work.