Software Deployment

There's no single answer that will work in all cases, but here are some suggestions.

1. Install the software from your repackaged installer.  Start a setup capture and take a "before" image.  Launch the software and enter the serial number manually.  Complete the setup capture to see what changed.  This may reveal a registry key or file where the serial number is being installed.  Add this file or registry key into your installer.
2. Check the vendor's web site to see if they indicate where the serial number is stored once installed.  This is pretty rare, but it sometimes happens.  Searching their KB for something like "change my serial number" sometimes gets a hit.
3. If the vendor's installer is an MSI, look in the Property table.  You may see an empty property value with a promising name, like "PRODUCT_KEY" or "ACTIVATION_CODE" or "SERIAL".  Create an MST and fill in the property with the serial number, then see if that works.

Unfortunately, there are some applications that use convoluted serial number storage schemes (e.g., an encrypted registry key or file) or which tie the application to the original hardware on which it was installed.  In a case like that, you'll be unable to include the serial in the package.