Heap4’s backing buffer is defined here: FreeRTOS-Kernel/heap_4.c at 1b8a4244bdd911f156ce5bc6f9f13dcde68228cb · FreeRTOS/FreeRTOS-Kernel · GitHub
To apply the attribute, you would define configAPPLICATION_ALLOCATED_HEAP to 1 in your FreeRTOSConfig.h, set configTOTAL_HEAP_SIZE to the size you want, and then define ucHeap.