Figuring out writing PlacementConstraint against `attribute:ecs.capability.external` attribute

Has anybody figured out for ECS how to write PlacementConstrain against attribute:ecs.capability.external attribute?

PlacementConstraints:
- Type: memberOf
Expression: attribute:ecs.capability.external

This is not valid Expression, yet since it has no value I really wonder how to write it

as it’s basically like a tag