We've added a new template tag to Buttondown's automation engine: {% random_number a b %}, which generates a random number between a and b.
You can use it like this:
{% load random_numbers %}
{% random_number 0 1 as number %}
{% if number %}
[Click this link](https://google.com?utm_source=a)
{% else %}
[Smash this link](https://google.com?utm_source=b)
{% endif %}