How do I programmatically "place" an order? When making a payment through the Commerce admin interface; the Orders do not get completed. I can see a Place Order button when viewing a cart Order; which seems to do the trick. I need to do this in code.

I know that i need to react to the the commerce_order.order.paid event but I am not sure what to do there.

I found this post: How do I programmatically update the order status? which suggests i need to "apply a transition". But the suggestion there doesn't work.

I opened a new question for this as I am specifically asking about "placing an order"; whereas that question was asking about simply changing Order status.
A little digging and I figured it out on my own:

Event handler for for this event: commerce_order.order.paid

and with this handler code: 

  public function paymentCompletedHandler($event) {
    $order = $event-&gt;getOrder();
    $order-&gt;getState()-&gt;applyTransitionById('place');
    return;
  }