Currently this integration uses the API key. Asana is deprecating the API key in the future in favor of other authentication mechanisms. In about a month's time, new Asana users will not be able to receive an API key, so they will be unable to use this integration.
While true Oauth is preferred, a stopgap to allow this integration to still work would be to support the Personal Access Token instead.
This would be a fairly easy change—would you be able to make that soon, or would you approve a pull request? Thanks!