How to get Windows username and use it in a system function

Question

I'd like to know how can I get someones Windows username, then store it somewhere so it can be used in a system function like this:

system("netsh wlan show profile their_wifi_name key=clear > C:\\Users\\their_username\\Desktop\\key.txt");

What do I need to use, something like popen() or? Is there even a function/way to to this?

I'm not looking to print out the Windows username but to use it in another function.