@lmathieu This is so annoying when Shopify changes their API but doesn’t provide any clear documentation for this. There is a guy who found a solution to this problem, you can check the solution detail here. Basically, whenever an order has been created in the store, shopify will automatically create a fulfillment order with status open.
Therefore, you have to find the fulfillment order id of your order, then you can call this endpoint to fulfilled your order