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');                                       
%%
handles.pushbutton1 = uicontrol('Style', 'pushbutton',...
                                            'Parent',Hauptfenster,...
                                            'String','Zum Unterfenster',...
                                            'Units','normalized',...
                                            'Position',[.08,.951,.1,.035],...
                                            'Callback',{@Zum_Unterfenster, Hauptfenster},...
                                            'Tag', 'pushbutton1');
setappdata(Hauptfenster, 'Handles', handles);                                         
function Zum_Unterfenster(~, ~, Hauptfenster)
Unterfenster(Hauptfenster)