How to create functioning % in gui?

I need help creating a functioning % push button in gui?

I don't even know where to begin. I have my basic design set and the calculator working properly, all but the percent sign. What is the coding to do so?

Below is what I have so far but it's not working properly.

% --- Executes on button press in percentage.
function percentage_Callback(hObject, eventdata, handles)
% hObject    handle to precentage (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
str=get(handles.inputtext,'string')
str=strcat(str,'%')
set(handles.inputtext,'string',str)

[Walter Roberson]

You have not defined what the % button should do.