What do you plan to connect using the header splitter?

You probably have an SD card reader in this PC.  It's a USB device and may connect to a 9-pin header on the motherboard. The header isn't marked in XPS 8900 motherboard pic, so you'd have to trace the cable from the reader back to the motherboard to find it. 

If you don't use the card reader, you may be able to use the header for a different device, or add a splitter for two devices.

There are also some PCI-e add-in cards that have an internal USB header, as an alternative to a splitter, assuming you have an available/compatible PCI-e open.

Planning to connect WiFi 7 adapter intel BE200 card

https://www.amazon.com/Bluetooth-Adapter-802-11be-Windows-Desktop/dp/B0CN17SC1Y/ref=sr_1_9?crid=3BPZZL5PV0XCT&keywords=asus+WiFi+7+PCI-E+Network+Adapter+BE200&qid=1707345599&sprefix=asus+wifi+7+pci-e+network+adapter+be200%2Caps%2C304&sr=8-9

I need connect 9pin USB cable to the motherboard in ordered for the Bluetooth to work

https://www.amazon.com/Splitter-Internal-Extension-Connector-Motherboard/dp/B0CCVGXM44/ref=sr_1_4?crid=1E6ON7JOA9B2V&keywords=9pin+usb+header+splitter&qid=1707346637&sprefix=9pin%2Caps%2C487&sr=8-4

Why do you need a 9-pin USB header cable to support Bluetooth on that card? 

Your Amazon link only says it needs a PCI-e slot, preferably a PCI-e x1 slot, and WiFi and Bluetooth drivers.  I didn't see any mention of needing a USB header connection.

BTW: Does your router/modem support WiFi 7..?  And is this card backward compatible with devices (eg, printers, cell phone, etc) that don't support WiFi 7?

Yes I have the following WiFi 7 Router from Linksys

https://www.linksys.com/mbe7001-tri-band-mesh-wifi-7-router/MBE7001.html

As you have done your research, the PCIe Wifi/Bluetooth combo card always needs a separate USB power for Bluetooth to work.

Yes, to power Bluetooth, you can use a USB splitter connecting to F_USB2 header on motherboard (#8).  Example of USB splitters you can use.

https://www.amazon.com/Rocketek-Motherboard-Extension-Splitter-Adapter/dp/B08SLQ63G7/ref=sr_1_10?keywords=usb+header+splitter&qid=1707365258&sr=8-10

https://www.amazon.com/SSSUWP-Motherboard-USB2-0-Extension-Splitter/dp/B0CJQ6JHND/ref=sr_1_8?keywords=usb+header+splitter&qid=1707365258&sr=8-8