The Automatic Trendline Detection indicator, based on the dicussion in "Trading Trendline Breaks" by Sylvain Vervoort, is available for download at here.
Once downloaded, from within the NinjaTrader Control Center window, select the menu File > Utilities > Import NinjaScript and select the downloaded file.
You can review the indicator's source code by selecting the menu Tools > Edit NinjaScript > Indicator from within the NinjaTrader Control Center window and selecting the indicator "AutoTrendLine".