I am trying to look for a simple way to design a winform with a combobox that has checkbox values in it to select multiple values.
But there are no free samples I could find
It sounds like what you really want is a checked listbox control or maybe even just a listbox. These controls do multi-select in a way that is more standard for Windows.
If you really need a combo box with checkboxes in it, here's an article on code project I used once.