Port forwarding for Transmission and Private Internet Access. Replace PIA-USERNAME and PIA-PASSWORD with your private internet access account user/pass.

set-port.scpt

set user to "PIA-USERNAME"
set pass to "PIA-PASSWORD"

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

set vpn_ip to do shell script "ifconfig tun0 | grep inet | awk '{print $2}'"

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 & ""

Download raw gist and open it with the AppleScript editor. Run this whenever you open Transmission.