https://wccftech.com/samsung-galaxy-s9-might-get-dockless-dex-support-in-android-pie/

Samsung DeX is Samsung’s desktop experience based on Android that appears when you connect eligible devices to a monitor. It allows for multiple windows and apps to be open at once with a special UI designed for a larger desktop screen. Until now, it required a dedicated DeX dock that had to be purchased separately. Starting with the Galaxy Note 9, it was possible to run it without a dock. The feature is soon to be ported over to the Samsung Galaxy S9 and Samsung Galaxy S9++. This means that all you need is a simple HDMI to USB Type-C cable to use Samsung DeX.

However, using DeX without a pad would only allow for five windows instead of the 20 windows that are part of the DeX with hardware. Today, XDA Developers discovered that the Samsung  Samsung Experience 10 beta supports DeX over HDMI. The hardware on the Samsung Galaxy S9+ and the Samsung Galaxy Note 9 are nearly identical. The smaller Samsung Galaxy S9 should also support DeX over HDMI, but not much is known about it yet.