You’ll need to find away to snag the tang so you can pull on it. It’s been awhile I’ve needed to fix a hinge or mount the VESA plate. I remember using a curved Upholstery Sewing needle like this the first and only time I lost the tang ;-{
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Then snaking in a strong cord so I can pull on it. The next is pressing on the spring latch moving it out of the way.
Here’s a vid showing the location of the spring [https://www.youtube.com/watch?v=eK55yuERL0k|Apple VESA Mount Adapter Unboxing and Install] Jump to 1:16 to see it.