async functions - where do I use 'esversion: 8'?

I'm trying to create an async function inside a behavior and I get the following message:

"async functions" is only available is ES8 (use 'esversion: 8').

I tried to add it to the behavior declaration in the model, however the message is still displayed, suggesting I didn't add it in the correct place.

Where do I add this?

Start by reproducing the reported async-function error inside a behavior and trace where the behavior declaration reads JavaScript options. Determine where the ES8 setting is documented or configured, then update the relevant guidance so users can place it correctly and confirm the error no longer appears.

Written by the indexing model from the issue text.