Fomo is installed on your site via an embed snippet, so an A/B test is a matter of including vs not including this snippet.
The exact instructions will depend on which tool you are using for your A/B test. However, here are a couple walkthroughs using some of the most popular A/B test tools.
These examples demonstrate a "Fomo" vs "no Fomo" A/B test. If you want to test out variants of Fomo settings, you can simply add a new Site in your Fomo account:
Each new site has its own embed snippet:
Feel free to use your second Site to build a variation of Fomo settings you want to compare, then include the different embed snippets as your "variants" in your A/B testing tool.