you need to use split view so you can access the code.
in the design side, select the date picker element.
On the code side, it will show the datae picker element code, there is a value attribute:
place your cursor between the quotes, and go to the bindings panel. In the bindings panel, expand the validated entries collection, select the corresponding element and click the bind button, this will add the validated entries binding code to the value attribute of the date picker element.