If possible, it is best to tell the user early on that you want them to record a video response.
If you are recruiting participants on the basis that they are willing to participate in the video survey, this is pretty straightforward - you just need to know that they have opted in and that they have the necessary hardware to deliver (internet, webcam, mic). Plotto works on desktop, mobile and tablet.
It is worth getting them to test the equipment (and their ability to use it!) up front as the most common mistake we see is the mic not working.
If participants have not been pre screened and the video option is being presented to all participants in a survey, we suggest telling them about the video element early on as this will increase the likelihood of them uploading usable content.