How to set the selected value of a dropdown list to a property of a thing.

This is covered in intro to Thingworx, which I highly recommend.

Any 'list of information' type widget has its selection tracked in the Data Source. You'll have All Data and Selected Row(s)

Use the Selected Row(s) either full row or individual item to bind to a service which sets the Property value.

Vice Versa you can take a value selected or a value returned from a Service and bind it to such a type of Widget to make it default to a certain value in the list of values in the Widget.