Triggering Additional Dropdowns

Here is a working example of how to do this:

load(
  local!box1,
  local!box2,
  with(
{
  a!boxLayout(
    label: "dropdowns",
    contents: {
      a!dropdownField(
        label: "box1",
        labelPosition: "ABOVE",
        placeholderLabel: "--- Select a Value ---",
        choiceLabels: {"Option 1", "Option 2"},
        choiceValues: {1, 2},
        saveInto: {local!box1},
        value: local!box1,
        validations: {},
        instructions: "select a value to show another box"
      ),
      a!dropdownField(
        label: "box2",
        labelPosition: "ABOVE",
        placeholderLabel: "--- Select a Value ---",
        choiceLabels: {"Option 1", "Option 2"},
        choiceValues: {1, 2},
        saveInto: {},
        validations: {},
        showWhen: not(isnull(local!box1))
      )
    },
    style: "STANDARD",
    marginBelow: "STANDARD"
  )
}
))

Thank you, Robert, this works perfectly.