What is the ruling on working for a trading platform like "Olymp Trade" that deals in currencies and metals, and is it permissible to appoint someone with money in exchange for a percentage of the profit?

We have no knowledge of the true nature of the transactions of the aforementioned company. However, it is probable that the transactions of such companies involve Sharia violations that render them invalid and prohibited, including financial leverage (margin trading) and contracts for differences or options. The Islamic Fiqh Academy has issued a ruling prohibiting them. As for giving money to someone who invests it for a known percentage of the profit, this is permissible and is called Mudarabah.