Launch SteamVR and go to SteamVR > Settings > Developer tab > Enable Direct Mode

Direct Mode ensures that your HMD is not recognized as a monitor. If you close SteamVR and the HMD display shows your desktop, this means that Direct Mode is not enabled. When Direct Mode is running properly, you will only see VR content in the HMD.

If you have trouble enabling Direct Mode, make sure that you've installed the most recent drivers for your GPU.

Make sure of two things:

1. Both your main monitor and your Link Box are plugged into the same GPU
2. Both your main monitor and your Link Box are plugged into functioning ports

In most cases, video ports that are side by side on your PC access the same GPU.

If you believe that the above are true, try:

• Using different ports of the same data type for each device (e.g. change Link Box HDMI cable from one HDMI port to a different HDMI port)
• Using a mini DisplayPort/DisplayPort cable or HDMI/DisplayPort adapter to plug the Link Box into your computer if the ports you regularly use are not detecting the device.
• Using a DVI or DisplayPort cable instead of HDMI, or vice versa.

Hit the X on SteamVR to quit, quit the Steam client, then restart your computer.