For me, it always auto copies when I click on the links under where it says "Share this photo". Then I just paste it wherever it needs to be.
If for some reason this is not the case for you, click on the text field to try and get a cursor, then do ctrl+a to select it, and ctrl+c to copy it.
If you can't get a cursor, try tabbing to the desired field with the tab key, then ctrl+a ctrl+c.