Well currently support for poster is not added in video in units , for that we would need revamp the video data in the get unit call in api and then handle this in app .
For now what we can do is to add single image as poster image for all your videos in app .
for that please goto src/pages/course-status/course-status.html file and edit it as per :
http://prntscr.com/lpy086
add the [poster] attribute in the video tag there .
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