It's very common knowledge that they met at the Kit Kat Club. Here are just a few examples of it being mentioned (you can easily find many more with a web search).

When Melania Knauss was introduced to Donald Trump at a fashion party at the Kit Kat Club in 1998, she told best friend Edit Molnar that the bombastic billionaire was “absolutely out of the question.”

https://nypost.com/2005/01/16/how-shy-fox-bagged-the-billionaire/

Before she met Donald Trump at the Kit Kat Club in Manhattan and stepped into a glitzy world of limos and penthouses, diamonds and caviar, Melania Knauss lived quietly.

https://www.chicagotribune.com/nation-world/ct-melania-trump-first-lady-20150930-story.html

The Kit Kat Club was founded by Ghislaine Maxwell.

In the late 1980s, she founded the Kit Kat Club. The club was a gathering place designed to bring together women from the arts, politics and society.

https://www.wduv.com/news/national/who-ghislaine-maxwell-companion-jeffrey-epstein/Uy4MDYotS6ugao3FuHKmtL/

She was invited to all the smartest parties. She is seven years older than me and when we were first introduced I was at once mesmerised and intimidated. She was an Oxford graduate who had founded the Kit Kat Club, an exclusive salon for the more intellectual sort of It Girl (if that isn’t a contradiction in terms).

https://www.dailymail.co.uk/femail/article-7453533/The-night-Ghislaine-tried-public-sex-lesson.html