XPS 8950, SSD addition

What's the intention here, use the new SSD as boot drive or as extra storage?  If as new boot drive, will it replace an existing SSD or HDD as boot drive?

I presume you're planning to install it in one of the two M.2 slots on the motherboard, correct?

Do you already have an NVME SSD in one of the M.2 slots?

Do you have a HDD installed?

You may need to reconfigure BIOS and Windows to use AHCI, rather than Dell's "standard" RAID setting, before installing the new SSD. Has to be done correctly or you'll make PC unbootable:

1. At the desktop, open Cmd prompt window, run as administrator
2. Copy-paste this command, which will start Windows in Safe Mode next time you boot:
   bcdedit /set {current} safeboot minimal and press Enter
3. Restart PC and tap F2 to open BIOS setup
4. Change SATA Operation from RAID to AHCI
5. Save the change and exit Setup. Windows will automatically boot in Safe Mode
6. Open Cmd again, as in step #1
7. Copy-paste this command, which will start Windows in Normal Mode the next time you boot:
   bcdedit /deletevalue {current} safeboot and press Enter
8. Reboot and Windows will automatically start with AHCI drivers enabled
9. Confirm that everything works properly

What comes next depends on your current drive config and intended use for the new SSD.

Configure BIOS and Windows to use AHCI, as above.

Then install new SSD in 2nd M.2 slot. You may need another M2x3 screw to secure new SSD, if Dell didn't put one in there already near the empty M.2 slot. 

Use imaging software to image the old SSD onto new one.  When you create the image on new SSD, you'll have to increase size of C: partition to use all that extra space or it will be wasted.  So make sure you know how your imaging software does that before you begin. Typically you copy all the other partitions onto new drive before copying the C: partition because only the last partition can be expanded, but check your software...

When that's done remove old SSD and set it safely aside.  Now confirm PC boots normally from new SSD.  

Once PC is booting from new SSD, you could reinstall old one and use the F12 menu to boot from new one. Otherwise it will always boot from old SSD.  Then use Windows Disk Management to initialize old SSD (ALL FILES DELETED!) so it can be used for routine storage. 

So be certain things are working ok before initializing the old SSD...