You'd have to add 16 rules on the field connected to the drop down.
Each one would have a condition.
For example, if the value of the field is "2", you'd set your text box to the "display" for value 2.
It seems to me it would be a lot easier to create an XML file with the options, add it to the form as a resource file, create a data connection to it, and then change the dropdown to use that data source. Then you can use a default value in your text field.