Can you be more thorough in describing what is pictured? I’m trying to intuit from your thread title and the image, so here’s my guess:
This is a Global Key Command trait on a piece in an At-Start Stack, and you’ve brought up the Expression Builder in the property match expression. Is that right?
EDIT: Looks like this is it. Confirmed that “Piece property” doesn’t appear, and it has no bearing on being about At-Start Stacks. Same thing when doing this on a GKC for a piece in the palette.
The piece is located in an at start stack: