Msi return value to keep control on the same dialog

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msi

Windows-Installer

Delphi

Hi, I am creating a dll custom action which is showing a message box taking the values from the property (stored in szBuffer1) table as below:

MessageBox(NULL, "This is a message box", szBuffer1, MB_OK);

I have attached this custom action to Next button of a dialog. This will pop a message box when user clicks on Next of that dialog. When user clicks on Ok button, I want to keep the control on the same dialog and the next dialog shouldn't be displayed. How should I achieve this (what value should I return when user clicks on 'Ok')? Please help