To unlock positions, i.e. for addition of opposite positions in one instrument, you can click on "Unlock" button in the window of the list of open positions and tick the necessary one(s) from the list. For instance, let's tick two opposite positions in #C-BRENT:
Let's unlocking the operation with "OK" /"Unlock selected" button to see the following:
Thus, after unlocking the long position in #C-BRENT with the volume of 142.28 and short position with the volume of 100.00 the long position with volume of 42.28 was created with the opening price equal to the one of the larger position (the linked orders Take Profit and Stop Loss also remained from the larger position).