Presuming that your subform and text field are called sfInfo and tfInfo, respectively, here's my solution to this problem:
btnAdd::click
sfInfo.instanceManager.addInstance();
xfa.resolveNode("sfInfo["+ (sfInfo.instanceManager.count-1) +"].tfInfo").rawValue = xfa.resolveNode("sfInfo["+ (sfInfo.instanceManager.count-2) +"].tfInfo").rawValue;