Custom Script - use ChoiceVar result in ObjectVar default parameter

[PieterL75]

Hey,

I have a Custom script where a user can select from a dropdown, and based on that dropdown, prefixes are selected in an ObjectVar.
The query_params takes the ChoiceVar with a $ in front, but I can't seem to find a way to use that in the 'default'

CONTAINERS = (
     ('prefixtagSite1', 'Site1),
     ('prefixtagSite2', 'Site2),
   )

   env_container = ChoiceVar(
      description="Location to select the Prefix",
      label='Location',
      choices = CONTAINERS,
      default = 'prefixtagSite1'
   )

   env_prefix = ObjectVar(
      description="Prefix to carve subnet out",
      label='Prefix',
      model=Prefix,
      query_params={ 'tag' : '$env_container' },
      default=Prefix.objects.filter(tags__slug='$env_container',custom_field_data__Utilization__lt=100)[:1]
      # the default is not taking the $env_container value...
   )

Is this possible ?

Pieter