There are two options for entering the highest and lowest price data. There is no better or worse and it depends on your objectives.
Option 1: The price during the duration of your trade
Here, you simply note the highest and lowest observed price points that the individual instrument made during the trade duration.
Option 2: The price beyond the trade exit
For this approach, you follow the price development beyond the trade exit. Here it is important that you determine a time window for how long you follow the price after you closed the trade.
If you trade the 4H timeframe, for example, and you generally hold your trades for a few days, you can choose to follow the price for a whole week (or two) and then write down the highest and the lowest price point that existed during this period.
If you trade the 15-minute timeframe and your average holding time is just 2 - 4 hours, you can track the highest and the lowest price for that trading day (or until the next day), even if you exit the trade before.
The most important aspect here is that you always choose the same approach so that the results you get are consistent and interpretable.