The "server file search" window does not support many of the custom command macros.
Basically only raw commands (commands sent to the server as-is) and %f, %d, %p style tokens are currently supported.
Due to the difference in logic/behavior while in the "server file search" a raw command-block is not performed using the same macro engine that's used in the main window browser panes.
This feature has slipped through the cracks for years
At the moment I am not sure if this feature needs to be abandoned or what, but in its current state its somewhat useless for anything outside of raw commands.