Here is some sample code to create a label with a rounded border:
var jsForm = solutionModel.getForm(controller.getName());
var jsRectangle = jsForm.newLabel("rectangle", 10, 10, 300, 100);
jsRectangle.borderType = solutionModel.createRoundedBorder(1,1,1,1, "#000000", "#000000", "#000000", "#000000", [10, 10, 10, 10], null);
controller.recreateUI();