appeventreceiver.cs

public SPRemoteEventResult ProcessEvent(SPRemoteEventProperties properties)
{
    SPRemoteEventResult result = new SPRemoteEventResult();

    using (ClientContext clientContext = TokenHelper.CreateAppEventClientContext(properties, useAppWeb: false))
    {
        if (clientContext != null)
        {
            Web hostWeb = clientContext.Web;

            var props = hostWeb.AllProperties;

            props["ModifiedFromAppInstalledEvent"] = "true";

            hostWeb.Update();

            clientContext.ExecuteQuery();
        }
    }

    return result;
}