Can I safely pressure can potato soup with milk in it?
According to USDA National Center for Home Food Preservation, no:


  Caution: Do not add noodles or other pasta, rice, flour, cream, milk or other thickening agents to home canned soups. If dried beans or peas are used, they must be fully rehydrated first.


From Penn State Extension:


  ...there are some commercially prepared foods that just cannot be reproduced safely by the home canner. Creamed soups are not suitable for home canning because their ingredients interfere with the proper transfer of heat during the processing step and can result in food borne illness.


From BUTTER AND MILK…WHY CAN’T THEY BE CANNED?
By Ruth Woods
UCCE El Dorado County Master Food Preservers 


  Look no further than a post by Dr. Elizabeth L. Andress, National Director of Home
  Food Preservation, University of Georgia Department of Foods and Nutrition. Dr.
  Andress, a Professor and Extension Food Safety Specialist, commented on several
  questions posed regarding the safety of home canning soups that contained butters and
  milks. According to Dr. Andress, there are no established safe procedures for canning
  dairy products. She echoed the stance that many personal internet sites that share
  canning recipes and information pose safety concerns: For a recipe to be safe, it needs to
  be thoroughly challenged in microbiology studies to confirm a safe product is achieved
  every time it is processed. Dr. Andress further explained that the “amount of heat that
  would have to be applied to kill harmful bacteria” that grows in dairy products in a
  processed jar held at room temperature would be “extremely detrimental to its quality.”
  She went on to say that, “Milk is a finely balanced emulsion of proteins in water. If the
  proteins are over-heated, they drop out of suspension and the milk separates.


There are commercially canned foods that contain dairy, but they use processes that are not available to home canners. To be safe, do not use dairy in recipes that will be canned at home.