Can you put a stop loss in after opening a trade…or does it need to be done at time of trading? If so, how pls 
stop loss is different in cfd to invest/isa so you first have to clarify what type of account you are looking at. However, in either you can create an order after opening a position. How you do it depends on the type of account.
Ensure you fully understand the different types of order and for stop loss you have stop and stop limit orders so be sure to understand them and the difference