Hi,
There is a typo in that code (that I apparently failed to fix): it should be newText("<p><a href='https://app.prolific.co/submissions/complete?cc=CGL9GN20'"+ GetURLParameter("id")+"' target='_blank'>Click here to confirm your participation on Prolific.</a></p> <p>This is a necessary step in order for you to receive participation credit!</p>") (ie. there should be a " after ', before + GetURLParameter)
Jeremy