Auto Fill Time Field

I suggest making a darte / time field called perhaps [Form Load Date / Time] and set the field properties to default to now.

Then make a formula Time of Day field called [Form Load Time of day] with the formula of ToTimeOfDay([Form Load Date / Time]).

Then have a form rule that says

when [Start Time] is blank 

Change [Start Time] to the Value in the field [Form Load Time of day] and set that to fire all the time, Uuncheck the box).

The result should be that it will auto populate, but can be overridden if needed.