Add a value to a Text='<%# Bind("txt_case_num") %>

I am trying to add a value to 
<asp:TextBox ID="case_numTextBox" runat="server" Text='<%# Bind("txt_case_num") %>' />
The value I would like to add is: generatePassword(6) which displayes a random case number. 
I can get it to show in the text box when I use Text='<%# generatePassword(6) %>'  /> but it does not write to the database
<asp:SqlDataSource ID="SqlDataSource_Test" runat="server" ConnectionString="<%$ ConnectionStrings:Officeclip_ConnectionString %>"  InsertCommand="INSERT INTO table Test(case_num) VALUES (@txt_case_num)"  SelectCommand="SELECT TableTest.* FROM TableTest"> <InsertParameters>
<asp:Parameter Name="txt_case_num" />
</InsertParameters> </asp:SqlDataSource>
How do I do this?
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