Add code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file as this will be wiped entirely when you update the theme.
https://gist.github.com/woogists/f24c41bac31f5ec695b98c6b06b3b19f
If you want to automatically add a product to the cart depending on the cart total use the following code:
https://gist.github.com/woogists/0928d4b3df4ba423ddb74317a7979527