This is a heavily modified version of cmaster11's SSH notifier, which he posted to Medium.com. His script was made to work with Notify17, which, at least for me, was inoperative. This version works with PushOver, which is still active.
Clone the repository
Create a PushOver account if you do not have one already
Create a new application. Choose a name that you will remember, upload a logo (If you want to), agree to the Terms of Service and the Guide to being friendly to the API.
Your application is now created! Copy the API key and replace YOUR_PUSHOVER_APP_TOKEN in the script with your application key.
Go back to your PushOver homepage and copy your user key. Replace YOUR_PUSHOVER_USER_TOKEN with your user key.
If you would like to whitelist your username from alerts, replace YOUR_USERNAME with your linux username. Otherwise, remove that entire if statement.
Edit the /etc/pam.d/sshd file and append the following line at the end:
session [default=ignore] pam_exec.so /bin/bash /opt/ssh-notify.sh
This has only been tested on Arch Linux, but it will most likely work on other linux distros. Test on Mac and Windows at your own risk.