You have a drop-down, bound to some node in either in your main DOM or a secondary DOM, which is popualted with your database values. To create a default selection, you need to either go to that specific node, to which the drop-down is bound, and set the default value there. If you are unable to set a default value on that node (i.e. because it is a secondary data source node), then you can use an open rule to set the value of that node when the form opens. Just pick some value that is in the list from the database and use that as the default value.