Hook to membership control

How can I get a reference to a textbox added to the CreateUserWizard? I have tried

Dim CUWS1 As CreateUserWizardStep = CType(CreateUserWizard1.FindControl("CreateUserWizardStep1"), CreateUserWizardStep)

Dim FirstName As TextBox = CType(CUWS1.FindControl("txtFirstName"), TextBox)
myCommand.Parameters.AddWithValue("@FirstName", FirstName.Text.Trim())
but the reference keep coming up NULL. Other attempts have failed as well.

Thanks 
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