Start ngrok at startup as a LaunchDaemon. Place in your /Library/LaunchDaemons dir

com.ngrok.startup.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>KeepAlive</key>
	<true/>
	<key>Label</key>

	<string>com.ngrok.onstartup</string>

	<key>ProgramArguments</key>
		<array>
		<string>/usr/local/bin/ngrokd</string>
		</array>

	<key>RunAtLoad</key>
	<true/>

	<key>StandardErrorPath</key>
  	<string>/tmp/ngrok.err</string>
</dict>
</plist>

nkgrokd.sh

#!/bin/sh
# launches ngrok w/ a ngrok configuration file. Opens the 'jenkins' tunnel
#
/usr/local/bin/ngrok start  --config /usr/local/bin/.ngrok2/ngrok.yml jenkins