Hey Lisa,
We will need to start by customizing the NewActivity screen.
Once in the Screen Designer for that screen click on Edit for <b>Status</b>
On step 2 of 6:
The condition is checking if they chose Completed from the list:
And the Enter Value into Field on Screen looks like:
In layman's term: 'Did User choose Completed? if TRUE then Update Due On to Current Date/Time.'
I hope this was clear, if not let me know and I can elaborate.
- Mortaza