addKey

# in ~/Library/LaunchAgents/com.user.addkeyonlogin.plist (replace <path to home>)

<?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>Label</key>
   <string>com.user.addkeyonlogin</string>
   <key>Program</key>
   <string><path to home>/.ssh/addkeyonlogin</string>
   <key>RunAtLoad</key>
   <true/>
</dict>
</plist>

# in ~/.ssh/addkeyonlogin (replace /path/to/key and p4ssword)

#!/usr/bin/expect
eval spawn ssh-add "/path/to/key"
#use correct prompt
set prompt ":|#|\\\$"
interact -o -nobuffer -re $prompt return
send "p4ssword\r"
interact

# run `launchctl load ~/Library/LaunchAgents/com.user.addkeyonlogin.plist`
# run `launchctl start com.user.addkeyonlogin`