The Pin function is the way to always have chats displayed at the top of the chat overview window.
You can use the Pin function for things like frequently used chats and chats you would like to reply to later.

In the chat overview window, hover your mouse over the chat that you would like to pin to the top of the list and click on the pin icon that will appear on the right side.

Alternatively, you can also pin a chat to the top of the list in the chat overview window by clicking the pin icon that is located on the side of the chat title in the center of the chat window.

Removing a pin is done by clicking the pin icon once more.