I don't understand what is the idea here. "Buy now" creates a new trade with Open Price equal to (or very close to) the Ask price. Let's say that Ask is 1.0020, so then Open Price will be 1.0020.
In "Buy now" you can obviously set Take-Profit in pips, let's say 7.5 pips. Why pips? Because we are all used to it. This value of 7.5 is nowhere recorded. This value is immediately and automatically converted to 0.00075 and added to the open price, so the actual price level for Take-Profit would be 1.00275.
Then, when the trade exists and you want to get Take-Profit, it's value will be 1.00275. It will not be 7.5. There is no need to add 7.5 to the Open Price, because Take-Profit already exists, it can be read and it's value is 1.00275. Take-Profit is one of the properties of each trade or order and it exists as a price level.
