Now you're making things complicated.
The code that I gave you already does what you need.
First of all, do not set OnClientClick in your aspx page, we add it instead in the code.
In the btnViewPrintVersion_PreRender function we specify a javascript that will open a window with the following URL : ~/member_statement_print.aspx?date=XXXX where XXXX will be the value of the selected item in the ddlStatementDates list.
If you need to change URL of the opening window, change it in btnViewPrintVersion_PreRender. If you need to change your XXXX value, change it in your ddlStatementDates list values.
Here it is simplified :