Call of Duty WW2 Zombies Groesten Haus Pack-a-Punch Easter Egg - How to Unlock Secret Pack-a-Punch Mystery Box in Groesten Haus (COD WW2):

Step 1 (Unlock Mystery Box): Shoot the 10 lanterns around the map. 4 are downstairs, 2 are outside the downstairs windows, 4 are upstairs. They make a raven sound when you shoot them and start glowing purple. This unlocks a secret room upstairs that contains the mystery box. The video shows all the lantern locations.

Step 2: Draw a Jack-in-the-Box from the Mystery Box. This is random but it's always one of the first few things you draw. Usually within the first 5 draws, but guaranteed within the first 10 draws.
Now throw that Jack-in-the-Box exactly where I throw it, on the wooden beam outside the mystery box room. Do it exactly as shown in the video.

Step 3: Collect 10,000 points by killing zombies. Then interact with the Piano in front of the mystery box room (upstairs) to pay 10,000 points. On PS4: hold Square-Button. On Xbox One: hold X-Button.

Now everything you draw from the Mystery Box will be pack-a-punched automatically! Unlike other maps there's no actual pack-a-punch machine. Instead, all weapon draws are immedately pack-a-punched. The best part: they continue to cost only 1000 points per box draw. By collecting "Max Ammo" pickups you can restore all your ammo.

Please note this does not work in the prologue. It only works on the Gresten Haus map (solo and co-op).