go to darktide’s settings in steam (cogwheel) and add that line as launch parameter.
i had the game crashing very rarely since i added the heapsize, that is only after lengthy scrolling through my huge weapons collection, but never during a fight due to running out of memory.
i could add even heapsize 4096 or so if it helped, but i haven’t seen this anywhere.
it’s a mystery though how fatshark hasn’t added such a basic fix as default heapsize yet to prevent such crashes, which are twice the fun in havoc.