there's already an explanation on how to use the command line, especially to save result in .txt format, the link:

https://github.com/AlexeyAB/darknet#how-to-use-on-the-command-line

to save time, i will provide the point which is might be helpful:

• To process a list of images data/train.txt and save results of detection to result.txt use:

• darknet.exe detector test cfg/coco.data cfg/yolov4.cfg yolov4.weights -dont_show -ext_output < data/train.txt > result.txt

might be late but maybe helpful to the others.