I managed to add TextView into LinearLayout by overriding OnCreate method. The snippet code below shows how.

public class SimpleActivity : Activity<SimpleViewModel>
{
    #region Constructors

    public SimpleActivity()
        : base(Resource.Layout.MainLayout)
    {
    }

    protected override void OnCreate(global::Android.OS.Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
                                                                   
        LinearLayout mainView = this.FindViewById<LinearLayout>(Resource.Id.Container);

        TextView txtView = new TextView(this);
        txtView.SetTextColor(global::Android.Graphics.Color.ParseColor("#FFFFFF"));
        txtView.SetTextAppearance(this, global::Android.Resource.Style.TextAppearanceMedium);
        txtView.Text = "test";

        mainView.AddView(txtView);
    }

    #endregion
}

Hope this helps.