MacOS Mojave or newer
Press Shift + Cmd + 5
It calls up a small panel at the bottom of your display.
Please choose the first one to the left.
On the right side is an "Options" button. It lets you choose where to save your screenshot. Please choose "Clipboard".
After that, your cursor will turn into a small camera icon. Click on the screen you wish to capture, and the screenshot is now copied to you clipboard.
Press the "PrtScn" button on your keyboard to copy the screen to the clipboard.
The print screen button may be labeled “PrtScn,” “PrntScrn,” “Print Scr,” or something similar. On most keyboards, the button is found between F12 and Scroll Lock.