CreateUserWizard - How to check if username exists before pressing create user button?

Hi,
I would like to trigger the validation controls that check if the user name, password and email fields of the CreateUserWizard, before the Create User button is pressed. This is because I want to avoid a postback that clears the fileupload control that I have also added to the wizard.

I tried adding a button that causes validation and belongs to the validation group, but this only fires the requiredfield validator controls.
Thanks for any advice.

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