Here is my current status for connecting my ultra wide Samsung monitor to my Surface Book /Surface Dock:
I ordered 2 cables for testing - one cable is a mini display port to display port; and the second is an active mini display port to HDMI. My monitor has a display port and 2 HDMI interfaces.
The first cable I received was the mini display port to display port and when I connected this it was a complete and immediate success!! The monitor switched over right away and had the correct resolution of 2560x1080. To say I was happy was an understatement. I installed the monitor drivers for the display port and all is as it should be.
The second cable I received was the active mini display port to HDMI. I disconnected the display port cable and plugged in this new cable and I had the same issue as with the passive mini display port to HDMI meaning it would not recognize the higher resolution so the monitor and my surface both stayed black. All of this was the same exact issue as before. I am not sure if the current monitor drive installed was interfering, but honestly, was not about to make more changes at this point. I went through the typical reboot, reset the surface dock, etc, but no luck.
If you only have an HDMI port, it may be worth trying the active mini display port to see if that will work. For my configuration, the display port seems to work the best.