Add the SaveRecord and DisplayExistingRecord fields to the form via Modifier
Open Sales Transaction Entry, open Modifier
Click on Layout > Show Invisible Fields. You'll see a bunch of fields surround the form. Drag SaveRecord and DisplayExistingRecord onto the form
Make them visible. In the screen shot below, one is visible and one is not.
Back in Dynamics, set security to the modified form.
EDIT 8/27/2016: In the form, click on Tools > Customize > Add fields to Visual Basic to make these new fields visible in the code behind
In VBA, add code to the DisplayExistingRecord_AfterUserChanged or the SaveRecord_BeforeUserChanged events.
The code below opens the Sales Date Entry form and populates a field. We could close the form also, if desired
EDIT 8/27/2016 After this is all over, carefully return to modifier and DRAG THESE FIELDS OFF THE FORM AGAIN. Note that this does not say 'delete the fields' (they may not ever be deleteable, not sure). Just move them off, out of the visible form. They'll still be available for you to code against.