I know you have to make a POST request, but I have no idea how to. I have searched the web but found nothing about this. Thanks in advance.
Steam API docs available here: https://steamcommunity.com/dev
In particular DeclineTradeOffer method: https://developer.valvesoftware.com/wiki/Steam_Web_API/IEconService#DeclineTradeOffer_.28v1.29
E.g.
http://api.steampowered.com/IEconService/DeclineTradeOffer/ including the tradeofferid in the POST data.