Dynamic choice list in custom variable on startup Sweet for ArcGIS

[ChristopherDawe]

I am developing an application Sweet for ArcGIS and would like to use custom variables that can be set as session variables to allow them to be pulled through to feature attributes. 

In the custom variable I have set it as a choice and trying to return the attributes from a layer within the map that a user can select. The layer is of site/survey boundaries and the attribute would be for project number so when a user starts up the application, they can select the project number from the list and this is then stored as a session variable that can be applied throughout.

The Arcade script i currently have is below, and running it in the test environment returns a feature set list that I would expect to see.

filter(FeatureSetByName($map, "NVC Survey Boundaries",['ProjectNumber']),"BoundaryType in ('Site Boundary')")

However upon startup, i can see that the custom variable is a drop down list but it is empty and nothing happens when clicking on the drop down. I presume i need to make the returned attributes into a list that can be leveraged by the custom variable but stuck on this part.