The pair struck at the office on Market Place at around 9pm on Saturday when they threatened staff and took cash from the till. No one was hurt.
The robbers fled through the rear of the building towards Oldfield Lane.
One of the men – who was carrying the knife – is described as Asian, around 6ft 1in tall and in his late teens.
He was of very stocky build and spoke with a local accent.
He was wearing a black woollen balaclava, a black knee length hooded parka-style jacket, thick gloves, black tracksuit bottoms with white piping on the right leg and trainers.
The second man is also described as Asian and in his late teens.
He was around 5ft 7in tall, of slim build, with short, black hair, a thin face and stubble.
He was wearing a white tracksuit top and also spoke with a local accent.
Anyone with any information should contact PC Ben Howley at Dewsbury CID on 0845 6060606 or Crimestoppers anonymously on 0800 555 111.