There is no official API for that. MT4 is not "pluginable". The tool you are talking about is not a plugin, it's a third party software that works on files used by MT4 (fxt and hst files).
MT4 is obsolete and no more developed, that's a weird idea to start investing time and money around it.
Hello, i'm not a developer (but i can hire someone). I would like to undestand how it could be possible to create a plugin (not mql4 EA, but additional features for backtester) similar to <> (but not related to tick datas) that add some advanced feature to the backtester tool.
Could you let me understand wich is the right way to start a commercial plugin to enance the backtest features?