Any C++ STEAM API's for recieving messages from friends ?

[LOW*]

I couldn't find anything for c++...

[LOW*]

Yeah, messages are something I need.
I found this : https://lab.xpaw.me/steam_api_documentation.html

Scroll down to IFriendMessagesService. That might be helpful. When I open link It says I can't access that page, I need a key.

I got that key, but I don't know where to type it...

(https://api.steampowered.com/IFriendMessagesService/GetRecentMessages/v1/)
That is the link for getting recent messages. I know that in these links if I want to enter my key i need to add to that link next : ?key=XXXXXXXXXXXXXXXX, but still I cant access the page. This might be it if it gives me info that I want.

I only want to recieve messages inside my app.
Link above would give me that as a JSON data.
That would be awesome. But I can't access the page.