Try something like this

void loop()
{
  RemoteXY_Handler ();

  uint32_t now = millis();
  static bool buttonWasPressed = false;
  static uint32_t buttonMillis = now;

  if ( RemoteXY.Unlock_Door == 1 )
  {
    buttonWasPressed = true;
    buttonMillis = now;
    digitalWrite(PIN_UNLOCK_DOOR, HIGH);
  }

  if ( buttonWasPressed && now - buttonMillis >= 3000UL )
  {
    buttonWasPressed = false;
    digitalWrite(PIN_UNLOCK_DOOR, LOW);
  }
}

Im using Arduino Uno and Esp 8266 Wifi Module to control a relay that is connected to a solenoid door lock.

Just wondering how can i add a delay to the button control on the smartphone that the button stay on high (Unlock Door) 3 seconds before going to low(Lock Door) again.
TQ

My Code:

/*
   -- New project --
   
   This source code of graphical user interface
   has been generated automatically by RemoteXY editor.
   To compile this code using RemoteXY library 2.4.3 or later version
   download by link http://remotexy.com/en/library/
   To connect using RemoteXY mobile app by link http://remotexy.com/en/download/                   
     - for ANDROID 4.5.1 or later version;
     - for iOS 1.4.1 or later version;
   
   This source code is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.   
*/

//////////////////////////////////////////////
//        RemoteXY include library          //
//////////////////////////////////////////////

// RemoteXY select connection mode and include library
#define REMOTEXY_MODE__ESP8266_HARDSERIAL_POINT

#include <RemoteXY.h>

// RemoteXY connection settings
#define REMOTEXY_SERIAL Serial
#define REMOTEXY_SERIAL_SPEED 115200
#define REMOTEXY_WIFI_SSID "RemoteXY"
#define REMOTEXY_WIFI_PASSWORD "12345678"
#define REMOTEXY_SERVER_PORT 6377

// RemoteXY configurate 
#pragma pack(push, 1)
uint8_t RemoteXY_CONF[] =
  { 255,1,0,0,0,13,0,10,13,1,
  1,0,20,37,21,21,2,31,88,0 };
 
// this structure defines all the variables and events of your control interface
struct {

    // input variables
  uint8_t Unlock_Door; // =1 if button pressed, else =0

    // other variable
  uint8_t connect_flag;  // =1 if wire connected, else =0

} RemoteXY;
#pragma pack(pop)

/////////////////////////////////////////////
//           END RemoteXY include          //
/////////////////////////////////////////////

#define PIN_UNLOCK_DOOR 2

void setup()
{
  RemoteXY_Init ();
 
  pinMode (PIN_UNLOCK_DOOR, OUTPUT);
 
  // TODO you setup code
 
}

void loop()
{
  RemoteXY_Handler ();
 
  digitalWrite(PIN_UNLOCK_DOOR, (RemoteXY.Unlock_Door==0)?LOW:HIGH);
 
  // TODO you loop code
  // use the RemoteXY structure for data transfer
  // do not call delay()

}