First, sauté the leeks in a few sprays of low calorie cooking spray then add them to the slow cooker. If you use an Instant Pot set it to Sauté.
When the leeks start to soften, add the ham and the potatoes.
Top up with the stock.
Set to Slow Cook for 4 hours, or until the ham is tender and the potatoes soft.
When everything is cooked, if you want the soup a bit thicker, roughly mash up some of the potato with a potato masher, just enough to thicken the soup slightly, then cook for another 30 minutes or so.