IPhone : How do I drag and drop a label to autocomplete textfield?

I want to drag and drop a label or something that will autocomplete the UITextfield. For example if I have an empty textfield and I drag a label with text "hello world" on top of the textfield, the textfield will have "hello world" as its text. Does anyone know how I can do this? Is a UILabel the best choice to use? or perhaps a UIButton could work?