I swap a lot used ECM's not new ones on older Chrysler's and never need the PIN. Is a used computer or new?
Powertrain Control Module Initialization:
When the PIN number is not available from the vehicle owner or a Chrysler Group Scan Tool(s) is not available this can be accomplished by writing the Vehicle Identification Number (VIN) into the PCM using an after-market scan tool with a "VIN Write Function".

This is an example without disconnecting anything except the battery once to swap the PCM. The vehicle have transponder Key:  http://tinyurl.com/hme3fa2

  Nope with transponder.
Can you try the following that works on older Chrysler's early's 2000 with transponder keys:
Disconnect the battery then  disconnect the ABS and WCM modules. Remove the old computer then install the used computer Connect the battery and write the VIN number on the new used computer when finish turn the ignition off, disconnect the battery and reconnect both ABS and WCM modules. Connect the battery, delete any codes try to start the engine.

                      Pretty sure your correct it's done through WCM at least for sure keys are done there with secret pin. Haven't replaced PCM yet but kinda remember PCM replacement along with key functions in WI-tech. Did you check if short term subscription with tech authority will allow PCM replacement through J-box if it isn't in your software. Autel has gotten cold feet about security functions since their lawsuit that's why I stopped updating in 2016.

 If you get a computer from the junkyard just install it. Turn the key on, do not try and start yet, connect your Autel tablet. Go into the engine computer under special functions and select VIN check, when the VIN is displayed it will be the wrong VIN. Simply enter the correct VIN and the tablet will write the VIN to the used module.