Bluetooth Core v5.3 Set Extended Advertising Data Command - bluetooth

Question

I'm trying to re-implement HCITool functions in my application. Currently trying to get the command "cmd" working so that I can set extended advertising data which will hold manufacturer data.
My first approach was to see how HCITool registers the command and so I came up with this:
hcitool -i hci0 cmd 0x08 0x0037 01 03 01 0C FFFF 12 34
I also tried it this way:
hcitool -i hci0 cmd 0x08 0x0037 01 03 01 0C FF FF 12 34
These gave me error code 1, which is saying the command doesn't exist so I'm guessing I didn't write it correctly. My first request is if anyone can explain how to correctly send this command in HCITool.
The following is how I am trying to implment it in my code. This code is just a lightly altered version of open source HCITool's approach.
static void cmd_manufacturer(int dev_id)
{
unsigned char buf[HCI_MAX_EVENT_SIZE], *ptr = buf;
struct hci_filter flt;
hci_event_hdr *hdr;
int i, opt, len, dd;
uint16_t ocf;
uint8_t ogf;
char arr[][20] = {"0xFFFF", "0x12", "0x34"};
if (dev_id < 0)
dev_id = hci_get_route(NULL);
errno = 0;
ogf = strtol("0x08", NULL, 16);
ocf = strtol("0x0037", NULL, 16);
if (errno == ERANGE || (ogf > 0x3f) || (ocf > 0x3ff))
{
return;
}
for (i = 2, len = 0; i < 12 && len < (int) sizeof(buf); i++, len++)
{
*ptr++ = (uint8_t) strtol(arr[i], NULL, 16);
}
dd = hci_open_dev(dev_id);
if (dd < 0) {
perror("Device open failed");
exit(EXIT_FAILURE);
}
/* Setup filter */
hci_filter_clear(&flt);
hci_filter_set_ptype(HCI_EVENT_PKT, &flt);
hci_filter_all_events(&flt);
if (setsockopt(dd, SOL_HCI, HCI_FILTER, &flt, sizeof(flt)) < 0) {
perror("HCI filter setup failed");
exit(EXIT_FAILURE);
}
printf("< HCI Command: ogf 0x%02x, ocf 0x%04x, plen %d\n", ogf, ocf, len);
if (hci_send_cmd(dd, ogf, ocf, len, buf) < 0) {
perror("Send failed");
exit(EXIT_FAILURE);
}
len = read(dd, buf, sizeof(buf));
if (len < 0) {
perror("Read failed");
exit(EXIT_FAILURE);
}
hdr = (void *)(buf + 1);
ptr = buf + (1 + HCI_EVENT_HDR_SIZE);
len -= (1 + HCI_EVENT_HDR_SIZE);
printf("> HCI Event: 0x%02x plen %d\n", hdr->evt, hdr->plen);
hci_close_dev(dd);
}
I run my application, turn advertising on and am using btmon to see the HCI events/commands. Btmon doesn't show my extended advertising data being set.
Anyone have any idea on what I can do to make this work? I think I'm not providing the data correctly to the function.
Thank you for the help.