Dy default you can't add option in dropdown list from jquery or java-script.
Because Asp.net checks each controls after postback.
You can change the page directive as follow
<%@ Page ... EnableEventValidation="false" %>
which will cause no validation on the server side which is not good thing.
If you want to achieve the above you can use html drop
down and use a hidden field
and get the selected value from hidden field
rather than drop down it will work.
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