Uses Private Internet Access REST API to set which port Transmission listens on, binds all Transmission traffic to the tun0 (typically OpenVPN) network interface, and then opens Transmission.

transmission-vpn-launcher.applescript

set user to "USERNAME"
set pass to "PASSWORD"

--set token automatically based on macaddress
set macAddHash to do shell script "/sbin/ifconfig en0 | /usr/bin/awk '/ether / {print $2}' | /usr/bin/tr -d ':' | md5 "

try
  
	set vpn_ip to do shell script "ifconfig tun0 | grep inet | awk '{print $2}'"
	--hack: must delete property before writing it but first run 'defaults delete' fails
	do shell script "defaults write org.m0k.transmission BindAddressIPv4 " & vpn_ip & ""
	do shell script "defaults delete org.m0k.transmission BindAddressIPv4"
	do shell script "defaults write org.m0k.transmission BindAddressIPv4 " & vpn_ip & ""
	set running_message to ""
	
	set vpn_port to do shell script "curl -d \"user=" & user & "&pass=" & pass & "&client_id=" & macAddHash & "&local_ip=" & vpn_ip & "\" https://www.privateinternetaccess.com/vpninfo/port_forward_assignment | grep -oE \"[0-9]+\""
	do shell script "defaults write org.m0k.transmission BindPort " & vpn_port & ""
	do shell script "defaults delete org.m0k.transmission BindPort"
	do shell script "defaults write org.m0k.transmission BindPort " & vpn_port & ""
	set login_message to ""
	
	tell application "Transmission" to activate
on error error_message number error_number
	
	display alert ("Error") ¬
		message error_message ¬
		& (" Error number ") & error_number
end try