Please follow the steps as outlined by the arrows in the attached picture. Basically, what you should do is
1) Select the Design View AppDesigner
2) Select the Component Whose Id You Want To Know
3) The highlighted text under the app.UIFIGUREin the Component Browser pane is the full ID of the selected component.
So if you selected a button whose text is "Select File" then in the Component Browser under app.UIFIGURE, the text app.SelectFile will be full id of the selected component.