How to detect the user'clicks

I'm trying to implement one functionality for combobox. but I'm facing one problem. Let's say we are using ASP.NET dropdown list in ASP.NET project. There are several items in that dropdownlist. We click the dropdownlist to open the dropdown. then, we click somewhere on the page. The dropdown will be hided automatically. I want to add this feature in my combobox too.
How do we know when the user clicks outside of the control?

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