function Hauptfenster
scrsz = get(0,'ScreenSize');
%%
Hauptfenster = figure('Position',[0.01*scrsz(4),0.065*scrsz(4),0.99*scrsz(3),0.895*scrsz(4)],...
                    'Units','normalized',...
                    'Resize','off',...
                    'MenuBar','none',...
                    'Name','Test Datenübergabe Hauptfenster',...
                    'NumberTitle','off',...
                    'Tag','figure1');
%%                
handles.PopupmenuString = [{'Auswahl 1'},{'Auswahl 2'},{'Auswahl 3'}];
handles.popupmenu1 = uicontrol('Parent',Hauptfenster,...
                                             'Style','popupmenu',...
                                             'String',handles.PopupmenuString,...
                                             'Value',1,...
                                             'Units','normalized',...
                                             'Position',[.19,.96,.2,.025],...
                                             'BackgroundColor',[1 1 1],...
                                             'Tag', 'popupmenu1');                                       
setappdata(0,'PopupmenuString',handles.PopupmenuString); 
%%
handles.pushbutton1 = uicontrol('Style', 'pushbutton',...
                                            'Parent',Hauptfenster,...
                                            'String','Zum Unterfenster',...
                                            'Units','normalized',...
                                            'Position',[.08,.951,.1,.035],...
                                            'Callback',@Zum_Unterfenster,...
                                            'Tag', 'pushbutton1');
                                        
function Zum_Unterfenster(~,~)
Unterfenster