Upload the video to Vimeo.com
While it’s uploading, set Privacy to ‘Hide this video from Vimeo’. This ensures nobody will be able to accidentally stumble upon it.
Wait till the video has been uploaded and processed (i.e. when you can actually watch it on Vimeo), then add it to your board on Niice as normal.
If the video is already uploaded on Vimeo, go to your video’s settings page, and in the Privacy settings, ensure it looks like this:
(Comments need to be turned on to embed the video successfully.)