There are two special fields for this in the Take Profit / Stop Loss section. You can set the desired profit level in the account currency or as a percentage of the account balance. You can also do the same with the risk.
For example, if you want to set a Take Profit order at such a level that your profit is $15, then you just need to enter $15 in the profit box so that the order price is automatically calculated.
The $ Risk, % Risk fields are duplicated in Quantity and Stop Loss. Using these fields, you can specify the risk for automatic calculation of the amount, or for automatic calculation of the Stop Loss level of the order. You can't automatically calculate both the Quantity and Stop Loss levels at the same time.