The scene has been written by the story's writer and former showrunner Steven Moffat; organiser Emily Cook told Radio Times:
Seeing as we’re all stuck in self-isolation with nothing to do, and given so many fans have got engaged with this now-global Day of the Doctor rewatch party, I thought it might be fun to create a new introduction video, inspired by the one which was shown in cinemas before the Anniversary Special was simulcast in 2013! I approached Steven Moffat with the idea and, amazingly, he jumped on board! He’s written a special scene which is utterly brilliant! Brand-new Doctor Who created remotely while we’re all in self-isolation. I really hope fans enjoy this! It's a real treat!Details of the scene, and who will present it, are being kept under wraps, but the writer has briefly returned to Twitter to celebrate the event, saying:
Hi. Steven Moffat here. Actual Steven Moffat, if there's still anyone bothering to pretend to be me. Only here for the Day Of The Doctor tweet-a-long, 7pm GMT, 21st March. #savetheday Disappearing after that BEFORE THEY GET ME!!!!
The worldwide "watchathon" takes place at 7:00pm GMT/3:00pm ET/6:00am AEDT/8:00am NZDT.