1. Create a new snippet and select Post excerpt as snippet position.
2. Choose after how many characters do you want to load the snippet, you can add 0 to load the snippet before the post excerpt, -1 to load the snippet after the post excerpt or any value, our system avoid to break the post excerpt.
3. Select which custom taxonomies you want to include or exclude (depending by the chosen custom taxonomy match value)
4. Put your code and click on Save button.
Here a videotutorial that will help you to load a snippet inside the WordPress post excerpt