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You can add HiddenField to the Gridview like that:

<asp:GridView ID="grdVw" runat="server" AutoGenerateColumns="false">
     <Columns>
         
          <asp:TemplateField>
              <ItemTemplate>
                   <asp:HiddenField ID="hdnID" runat="server" Value='<%# Bind("ID") %>' />
               </ItemTemplate>
           </asp:TemplateField>
     </Columns>
</asp:GridView>

And you can get the HiddenField Value in C# as below:

comboBoxID.SelectedValue = ((HiddenField)grdVw.SelectedRow.Cells[0].FindControl("hdnID").Value;