I actually figured out how to do this. You need this expression: {{entry.author.[0].family}}
However, I still have to figure out how to create APA style in-text citations where it automatically becomes “FirstAuthor et al.” when there are more than 2 authors. It should be fairly easy to achieve this. If I had some javascript knowledge I would fork the source code myself but I really don’t have the time to figure everything out right now. I hope the author adds this feature in the future.