You can use my script here: https://spaghettidba.com/2014/09/05/database-free-space-monitoring-the-right-way/

It can be used in a SQL Agent job step, scheduled to run every 5 minutes (or whichever interval is appropriate for you) and send an email to you if the threshold is exceeded.

In the script I linked, the space available threshold is set at 20 (toward the end of the script): you can set it at 15 if that works for you, but be advised that 20% is a well established best practice.