Question :
In the latest versions of macOS any screenshot I make is no longer immediately saved to the Desktop, but instead I get a popup screen with a preview asking me what to do with the screenshot… but there if I click on the Export button there is no option to export to the Whatsapp.app and it’s impossible to drag&drop the preview into the Whatsapp.app either…

What is te easiest way to get the screenshot into Whatsapp ?

Answer :
The easiest option would have been to just drag&srop the Screenshot preview… but as you already mentioned, that’s impossible.
There are 2 options to get to your desired result as quickly and easily as possible :

Option 1 : using Preview.app
– on your Mac, open the Whatsapp.app and start a new message, no need to typ anything in it yet, just make sure it’s ready
– on your Mac, create a screenshot by pressing the [CMD]+[SHIFT]+[4] key combo
– when the screenshot preview window opens, click the Export button (square with an upward pointing arrow – top right, next to the Done button)
– in the pulldown menu that appears, click Preview
– now the Preview.app opens the screenshot, so in Preview.app click the document name Untitled
– in the document info pulldown menu, set Where: to Desktop
NOTE : you need to (temporarily) save the screenshot anywhere on your Mac before macOS lets you move it to Whatsapp
– then, click on the document name Untitled again, and a document icon will appear left-next to Untitled
– drag that document icon into the Whatsapp.app and the screenshot will appear in your (already open) message
– now, typ any additional text for your message and send it out
– then close the screenshot from the Preview.app
– when you get a window asking you to save screenshot Untitled, click the Delete button since you proably don’t need to keep it
NOTE : no need to save the screenshot on your Mac if you don’t have any use for it – even if you need it later-on, you can get it back by taking it out of your Whatsapp message

Option 2 : the old style
– on your Mac, open the Whatsapp.app and start a new message, no need to typ anything in it yet, just make sure it’s ready
– on your Mac, create a screenshot by pressing the [CMD]+[SHIFT]+[4] key combo
– when the screenshot preview window opens, click the Done button (top right)
– now, drag&drop the screenshot from your Desktop into the Whatsapp.app and the screenshot will appear in your (already open) message
NOTE : if you have a lot of windows open, it will be faster to long-click on the Finder icon bottom-left in the Dock, select New Finder Window from the pulldown menu, click Recent in the menu on the left ot the new Finder window that opens and find your latest screenshot on top and drag&drop it into Whatsapp.app from there
– now, typ any additional text for your message and send it out
– then drag&drop the screenshot to the Trash (or Bin) to delete it (either from the Desktop or from the Recents folder if that’s where you took it from)
NOTE : no need to save the screenshot on your Mac if you don’t have any use for it – even if you need it later-on, you can get it back by taking it out of your Whatsapp message

That’s it !

enjoy 😉