Stuck on a dynamic form rule regarding current user and date

Sorry... I figured this out. Or at least a workaround I'm happy with. I can't find how I can delete my original question, or I'd just delete it. Sorry for the bother.  All good. As you were. Nothing to see here. :)


Original Message:
Sent: 11-30-2019 01:05
From: Joe Hargrave
Subject: Stuck on a dynamic form rule regarding current user and date

I think I'm making something more difficult than it is, and I apologize.  Maybe I just need sleep, or maybe this is the wrong way to go about this.  Here's what I'm trying to accomplish:

When a task is assigned to a user, I have adjacent fields that I want to auto-populate with 'Assigned By' (current user) and 'Assigned On' (current date).

My trouble is that I can't figure out how to utilize the act of assigning a user to the task as a trigger.  I want to essentially say 'when user (in the assigned user slot) is no longer blank' then ... action.  The action part is easy.  I got that.  I just can't figure out how to make a User field being populated initially as the trigger.  

The workaround I came up with would be to require the user assigning the task to check a box when the assignee is chosen. Then the trigger is simply the checkbox being checked.  But that seems like a silly thing to make people do.  I would prefer the 'assigned by' (current user) and 'assigned on' (current date) be filled by simply choosing a user in the 'assign to' field.

What am I missing?  Again, I apologize if this is super elementary and I'm missing something really obvious.
THANKS!

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Joe Hargrave
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