In there you need to define the Handler for the DatePicker.
The action you define there will be called every time you select a date.
Basically what you probably need to do is to assign the variable you had on your initial input (before turning it into the date picker) to the SelectedDateTime, that is the Date you choose in the datepicker.
You can check this link for more info https://success.outsystems.com/Documentation/11/Developing_an_Application/Design_UI/Patterns/Using_Mobile_and_Reactive_Patterns/Interaction/Date_Picker