Need to pass the selecteditem of a dropdownlist as __EVENTARGUMENT

I need to pass the selecteditem of a dropdownlist on the client side to c# code behind, I've been trying to use:
string ctrlArg = Page.Request.Params.Get("__EVENTARGUMENT");

but it come back null or blank, anyone know how to set the __EVENTARGUMENT variable using javascript?
 

 
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