Send system screenshot to Yoink.app from Eternal Storms Software. The default hotkey is ⇧⌘4, the same as the default system shortcut for screenshots.

Note: Default behavior is to send screenshot both to Yoink and to save it in Documents/Yoink (done by the workflow) and on your Desktop (done by the system, unless you configured it otherwise). If you want to disable saving on the Desktop, go to "System Preferences > Keyboard > Shortcuts > Screen Shots" and uncheck "Save picture of selected area as file".