StartActivity Not working Monogame

I'm trying to open a webpage in my Monogame app, but it is saying "The name 'StartActivity' does not exist in the current context"

My code is :

var uri = Android.Net.Uri.Parse ("https://www.google.com/");
Intent launchBrowser = new Intent (Intent.ActionView, uri);
StartActivity (launchBrowser);

And i am currently calling:

using System;
using System.IO;
using System.Text;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Input.Touch; 
using Microsoft.Xna.Framework.Media;
using Microsoft.Xna.Framework.Content;
using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Content.PM;
using Android .Content .Res ;
using Android.Views;
using Android.Widget;
using Android.OS;