Getting and Setting MSI Properties from InstallScript

According to InstallShield documentation, getting MSI properties can be done using MsiGetProperty() function:

MsiGetProperty(hMSI, "USERNAME", svName, nvSize);

However, the documentation does not say where to get the value passed throug the hMSI parameter. You discover it only when you start typing. IntelliSense proposes ISMSI_HANDLE as the default value, so:

MsiGetProperty(ISMSI_HANDLE, "USERNAME", svName, nvSize);