The final step before confirming your trade is selecting the quantity which can be done via the slider or manually entering it in the Qty box.
This will also reveal to you your trade metrics:
Initial Margin: The amount of capital you need to put up to place your trade on the orderbook.
Trade Value: Is the value of the assets you are purchasing: it is = quantity x price.
Maintenance Margin: Is how much capital you need to keep this position open once the trade has been executed.
To finalise your trade simply click the yellow buy/sell button at the bottom!