You have to define the price level at which the stop order will be activated. In case of a buy stop order, your stop price must be above the market price.
Then, set the limit price — the maximum price, which you are willing to pay for this cryptocurrency.
Finally, enter the amount of cryptocurrency you want to buy, and click Buy.
In the popped-out window, click Confirm.
This stop order will appear in the Open Orders tab and will be executed as soon as the market price reaches the specified conditions.