Hi MikeB,
I believe you are referring to the "Default Value" option on Step 1of5 for the "datepicker" properties, however you are correct this is where you would manually input the value you want.
I provided a screenshot below of the how I filled mine out, I tested it after, as soon as I load this screen, the date populates with Jan-01-2014. Technically, you can also do this as you said, from the "OnScreenLoad" function. Simply add an "Enter Value on Screen" action, and 'type in' your date here, this will do the same as using the default value in the datepicker properties, but would require a bit more work. If you do have an action result for this value, then you would likely need to add it using the "OnScreenLoad" function.