How to save the changes done on the property page when user clicks APPLY Button

#include "precomp.h"
#include "debug.h"
#include "resource.h"
#include "globals.h"
#include "devmode.h"
#include "oemui.h"
// This indicates to Prefast that this is a usermode driver file.
__user_driver;
////////////////////////////////////////////////////////
//      INTERNAL MACROS and DEFINES
////////////////////////////////////////////////////////
typedef struct _tagCBUserData
{
    HANDLE          hComPropSheet;
    HANDLE          hPropPage;
    POEMUIPSPARAM   pOEMUIParam;
    PFNCOMPROPSHEET pfnComPropSheet;
} CBUSERDATA, *PCBUSERDATA;

////////////////////////////////////////////////////////
//      INTERNAL PROTOTYPES
////////////////////////////////////////////////////////
LONG APIENTRY OEMPrinterUICallBack(PCPSUICBPARAM pCallbackParam, POEMCUIPPARAM pOEMUIParam);
LONG APIENTRY OEMDocUICallBack(PCPSUICBPARAM pCallbackParam, POEMCUIPPARAM pOEMUIParam);
LONG APIENTRY OEMDocUICallBack2(PCPSUICBPARAM pCallbackParam);
INT_PTR CALLBACK DevicePropPageProc(HWND hDlg, UINT uiMsg, WPARAM wParam, LPARAM lParam);


////////////////////////////////////////////////////////////////////////////////
//
// Initializes OptItems to display OEM device or document property UI.
//
//Added the below line for Warning Removal : warning C4100: 'pOEMUIParam' : unreferenced formal parameter
#pragma warning( disable : 4100 ) 
HRESULT hrOEMPropertyPage(DWORD dwMode, POEMCUIPPARAM pOEMUIParam)
{
    HRESULT hResult = S_OK;
    VERBOSE("hrOEMPropertyPage entry.\r\n");
    switch(dwMode)
    {
        case OEMCUIP_DOCPROP:
          //  hResult = hrDocumentPropertyPage(dwMode, pOEMUIParam);
            break;
        case OEMCUIP_PRNPROP:
          //  hResult = hrPrinterPropertyPage(dwMode, pOEMUIParam);
            break;
        default:
            // Should never reach this!
            ERR("hrOEMPropertyPage() Invalid dwMode");
            SetLastError(ERROR_INVALID_PARAMETER);
            hResult = E_FAIL;
            break;
    }
    return hResult;
}
////////////////////////////////////////////////////////////////////////////////
//
// Adds property page to printer property sheet.
//
HRESULT hrOEMDevicePropertySheets(PPROPSHEETUI_INFO pPSUIInfo, LPARAM lParam)
{
    LONG_PTR    lResult = 0;
    VERBOSE("hrOEMDevicePropertySheets entry\r\n");
    // Validate parameters.
    if( (NULL == pPSUIInfo)
        ||
        (PROPSHEETUI_INFO_VERSION != pPSUIInfo->Version)
      )
    {
        ERR("hrOEMDevicePropertySheets() ERROR_INVALID_PARAMETER.\r\n");
        // Return invalid parameter error.
        SetLastError(ERROR_INVALID_PARAMETER);
        return E_FAIL;
    }
    // Do action.
    switch(pPSUIInfo->Reason)
    {
        case PROPSHEETUI_REASON_INIT:
            {
                PROPSHEETPAGE   Page;
                // Init property page.
                memset(&Page, 0, sizeof(PROPSHEETPAGE));
                Page.dwSize = sizeof(PROPSHEETPAGE);
                Page.dwFlags = PSP_DEFAULT;
                Page.hInstance = ghInstance;
                Page.pszTemplate = MAKEINTRESOURCE(IDD_DEVICE_PROPPAGE);
                Page.pfnDlgProc = DevicePropPageProc;
                // Add property sheets.
                lResult = pPSUIInfo->pfnComPropSheet(pPSUIInfo->hComPropSheet, CPSFUNC_ADD_PROPSHEETPAGE, (LPARAM)&Page, 0);
            }
            break;
        case PROPSHEETUI_REASON_GET_INFO_HEADER:
            {
                PPROPSHEETUI_INFO_HEADER    pHeader = (PPROPSHEETUI_INFO_HEADER) lParam;
                pHeader->pTitle = (LPTSTR)PROP_TITLE;
                lResult = TRUE;
            }
            break;
        case PROPSHEETUI_REASON_GET_ICON:
            // No icon
            lResult = 0;
            break;
        case PROPSHEETUI_REASON_SET_RESULT:
            {
                PSETRESULT_INFO pInfo = (PSETRESULT_INFO) lParam;
                lResult = pInfo->Result;
            }
            break;
        case PROPSHEETUI_REASON_DESTROY:
            lResult = TRUE;
            break;
    }
    pPSUIInfo->Result = lResult;
    return S_OK;
}

INT_PTR CALLBACK DevicePropPageProc(HWND hDlg, UINT uiMsg, WPARAM wParam, LPARAM lParam)
{
	HWND hWndVal;
	hWndVal = GetDlgItem(hDlg, IDC_EDIT1);
	char strVal[20];
	int Val_len = 0;
	LPWSTR buffer = NULL;
	
	
    switch (uiMsg)
    {
	
		case WM_INITDIALOG:
	
			
			GetDlgItemText(hDlg,IDC_EDIT1, buffer,2);
			if (buffer != NULL)
			{
				SetWindowText(hWndVal, (LPCWSTR) buffer);
			}
			
			return TRUE;
        case WM_COMMAND:
            //if the textboxes are changed
			if (HIWORD(wParam) == EN_CHANGE)
			{
				SendMessage(GetParent(hDlg), PSM_CHANGED, (WPARAM)hDlg, 0);
			}
            return TRUE;
        case WM_NOTIFY:
            {
		NMHDR* pnmh = (NMHDR*)lParam;   
            if (pnmh->code == PSN_APPLY)   
            {   
				return TRUE;   
            }
		if (pnmh->code == PSN_KILLACTIVE)   
		{
		SetPrinterData(hDlg, OEMUI_VALUE, REG_DWORD, (PBYTE) (GetWindowText(hWndVal, (LPWSTR) strVal, 20)), sizeof(DWORD));
			
		GetWindowText(hWndVal, (LPWSTR) strVal, 20);
		Val_len = GetWindowTextLength(hWndVal);
			if (Val_len > 1)
			{
				//Error Enter value between 1 to 9
				MessageBox(hDlg, L"Enter value between 1 to 9", L"TESTING", MB_ICONWARNING | MB_OK);
			}
			else
			{
				SetWindowText(hWndVal, (LPCWSTR) strVal);
			}
			
			}
            break; 
            }
            break;
    }
    return FALSE;
}
#pragma warning(pop)