Scripting

The short answer to this is you can't, msi is open and insecure. The long answer is to create a property in the msi like VALIDATION_CHECK then create a custom action that makes sure the value is correct then add a dialogue allowing the user to input the password when it runs. The problem with this is anyone with an msi editor can remove the custom action. As I said msi is open and insecure. A more secure way would be using something like installsheild to create a password protected exe installer with the msi wrapped. A simpler method, depending on what you are trying to do is zip the msi in a password protected archive.