If you are on WIndows, pull up the quest, then press the Windows Button + Print Screen button
(May be labelled as PRT SCR, it's near the Delete button), or use the Xbox Gamebar (Windows Button + G)
On Mac, press Command + Shift + 3
On tablets, usually pressing the power/home button + volume usually works!
After you do that, you can upload the screen shot. Mobile, there will be an upload button on the bottom right corner. On the desktop,
Press that upload button and find the screenshot in your files