Diagnosis
The VESA mount interface is hidden behind the removable stand neck. The monitor is 100% VESA compatible (100x100mm pattern).
How to Fix It
To expose the VESA mount, you must remove the stock stand. Lay the monitor face down on a soft cloth. Firmly grip the stand neck where it meets the monitor. Press the quick-release latch (a small plastic tab) on the underside of the neck joint. While holding the latch, pull the neck straight down and away from the monitor. The four VESA screw holes and a plastic spacer guide are now visible.
If That Didn't Work
If the neck does not detach, you are not pressing the quick-release latch correctly. Do not force it. Re-examine the joint; the black plastic latch is clearly visible on the bottom side. Press it fully inward toward the monitor screen.
Heads Up
You must use the four screws provided with your VESA mount, not the screws that came with the Dell stand. The correct screw type for the S2421HS is typically M4 x 10mm, but always follow your mount's instructions. Overtightening can damage the monitor's internal structure.