CreateUserWizard Control: add a function when clicks "Next" button

Hi:

I have a createUserWizard control, I added an extra section at the begin of the registration, such as FirstName, LastName, AuthorizationCode, after user clicks "Next" button, then go to the default section, at last is the success create user section.

Now, after user inputs some data on that first section and clicks "Next" button, I need write some code to check if the AuthorizationCode matches the data in database. If not match, pop up an error msg, and just not let the use go to next default section.

Can you let me know how to do that. I even don't know where to add the code. 

  <WizardSteps>

            <asp:WizardStep ID="WizardStep0" runat="server" Title="Extra Information">

                &nb ...