Use Primary Order for placing your buy order and Conditional Close for placing your sell order.
Assume that you want to buy 1 bitcoin once the price reaches 18,500 dollar and sell it at 20,000 dollar.
You must set the limit price in primary order section at 18,500 dollar and the limit price in conditional close section at 20,000 dollar.
Once the price reaches 18,500, your buy order is executed and a sell order at 20,000 dollar will be created.
Visit the link below for more information.
Kraken - Other order options