VESA Mount Adaptor for 2019 Retina 4K 21.5-inch monitor

+1 for the Very Wise Toad. There are third-party solutions out there but is it "buyer beware." Some are poorly identified by compatible iMacmodel. People have ordered what they thought would work with a 2012 o later iMac but received a unit that only fit the pre-2012 stand configuration.

Starting in Late 2012, the VESA iMac became a separate product instead of a built-in system with quick-detachable stand.

Buy iMac with Built-in VESA Mount Adapter - Apple

Since the stand can't be removed from the iMac you'll have to search for a mount that will take the iMac and stand. A search of the internet should turn up some possibilities. I remember another inquiry over a year ago and some possibilities were found.