"Stop" is one of the order types in the dropdown menu. After you open your long you would enter a Stop Sell for the same amount as your long (or for less if you only want it to partially close, or for more if you want to automatically flip into a short when the Stop is triggered).

You can use the Stop order type even if you don't already have a position open - the meaning of it is "When the price reaches this trigger value, immediately do a market order", so you can use it to open a position as well as close one.