Rains on page until correct data

Hello froum. I have a custom page

Page custom UserPage UserPageLeave

On UserPageLeave, I need to control if $User, $Passwd and $Mail variables contain correct values as:



    ${NSD_GetText} $UserText $User

    ${NSD_GetText} $MailText $Mail

    ${NSD_GetText} $PasswdText $Passwd

    ${If} $User = ""

    ${OrIf} $Mail = ""

    ${OrIf} $Passwd = ""

        MessageBox...

    ${EndIf}

On ***, I need to remain on current page until the variables contain correct values. Now, I show a message box but when the user click on this message box, this installer show the following page.

Can you help me, please?
Thanks for all.