You don't need to read KML, you just have some more informations on some charcters form the start and the world if you read it. Both stories are in the same universe, taking place around the same time. Some characters are in both stories.
Emma, Josh's sister, works as a secretary for Keith Pittman. Keith Pittman is the owner of the production firm Chase is shooting a movie for during.
Grayson, Chase's brother, is friends with Keith.... and so on. Those are information that are nice to know, but not necessary to read and understand the story.
You don’t have to read KML, they are just in the same universe, I think the author added Keith in it as a little Easter egg
I don't think that qualifies as a cameo/easter egg, because that party happens in KLM too. It shows the same party from both stories in a different point of view. Check chapter 14 of KLM.
*KML
How is this related to KISS ME LIAR? I didn’t finish that one because the ML annoyed me. Do I have to read it to understand this story or not? Or are these just new characters and a new story in the same universe?