Windows installer, access a custom property inside a custom action in C++

I have a C++ application in Visual Studio 2010 and I have a Windows Installer (i.e. setup project) to install it. I want to be able to invoke the installer like this:

Setup1.msi MYPROPERTY=MyValue

And then be able to extract the value "MyValue" from the property from within my custom action. I tried to get it working by following this tutorial(C++ custom actions) and this tutorial (passing arguments to custom actions, but in C#) combined with some MSDN searches to get this code:

#define WINDOWS_LEAN_AND_MEAN
#include <Windows.h>
#include <msi.h>
#include <msiquery.h>
#include <stdio.h>

BOOL APIENTRY DllMain(HANDLE, DWORD, LPVOID) {
    return TRUE;
}

UINT APIENTRY InstallCustomAction(MSIHANDLE install_handle) {
    static const wchar_t* kPropertyName = L"MYPROPERTY";

    //auto msi_handle = MsiGetActiveDatabase(install_handle);

    DWORD n = 0;
    //auto result = MsiGetProperty(msi_handle, kPropertyName, L"", &n);
    auto result = MsiGetProperty(install_handle, kPropertyName, L"", &n);
    wchar_t* value = nullptr;
    if (result == ERROR_MORE_DATA) {
        ++n;
        value = new wchar_t[n];
        //result = MsiGetProperty(msi_handle, kPropertyName, value, &n);
        result = MsiGetProperty(install_handle, kPropertyName, value, &n);
    }

    if (result == ERROR_SUCCESS) {
        wchar_t buffer[128];
        swprintf_s(buffer, L"n = %d, value = %s", n, value);
        MessageBox(nullptr, buffer, L"CustomAction", MB_OK);

    } else {
        MessageBox(nullptr, L"Error reading property", L"Error", MB_OK);
    }
    delete value;
    //MsiCloseHandle(msi_handle);
    return ERROR_SUCCESS;
}

I'm following the C# tutorial exactly in terms of the IDE (I have Entry Point set to InstallCustomAction and Custom Action Data set to /MYPROPERTY=[MYPROPERTY]) The custom action fires correctly but I don't get the parameter.

With the code as-is, I get n=0. If I use the msi_handle from MsiGetActiveDatabase I get an error (i.e. MsiGetProperty returns something other than ErrorSuccess).

How can I get the property that the user passes in on the command line from within my custom action?