Sorry, my bad.
In Traditional Web, the list of selected options is defined by the Check property.
So your list of options should have a variable of type Boolean filled with True for the ones selected and False for the ones not selected. You then fill the Check property with that attribute from the list you fill in Source Record List.
You can do this by adding a calculated attribute to your query called IsSelected where you fill it to True or not, depending if you want to have selected.
Kind Regards,
João