Could you use some conditional logic to hide the button until say a single radio button is clicked?  It’s not “automatic”, but basically they watch the video then click then radio field which makes the next button appear.  It doesn’t force the watching of the video per se, if that’s what you’re trying to accomplish, of those that want to circumvent the system.   But it may keep the honest one’s honest. 

Thanks for the reply - this is what I have been using in absence of a better solution. Maybe what I should do is figure out how to record the time in a hidden field and the the time on completion - and calculate what length it is and if not long enough for the video they can’t progress.

Have you solved this yet?  I am working on a solution myself, I will let you know if I come up with anything. 

I have had some progress - I used this code:

var tag = document.createElement('script'); tag.src = "https://www.youtube.com/iframe_api"; var firstScriptTag = document.getElementsByTagName('script')[0]; firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);   var player; // when video ends function onPlayerStateChange(event) { if (event.data == 0) window.location.href = 'https://www.duckduckgo.com'; } 

This code is put into WPCode plugin. In the form I put in this iframe into an html field:

<iframe id="player" width="640" height="360" src="https://www.youtube.com/embed/ZdP0KM49IVk?enablejsapi=1" frameborder="0" allowfullscreen />

This worked well as long as the form was on a page (wouldnt work on preview as the WPCode didnt load into it) - I then tried to use the advice on inserting a value into a hidden field by @Bobby Clapp - obviously changing the field key to the hidden field

// when video ends function onPlayerStateChange(event) { if (event.data == 0) $("#field_6j2syb").val('video-played'); }

but nothing happened - is the code above correct for using javascripot to insert a value into a field? Any help would be appreciated.    

Okay so a bit more digging with the help of all above and some posts on stack overflow. First thing was I needed to call jQuery - so changed the "$" to "jQuery". The below code is running on WPCode - haven't tested it directly in the form yet. Issues are I can automatically populate a text field but this doesn't seem to be recognised as an entry on conditional logic for the submit button - am not sure why as if I type the phrase it works fine - just not if filled via javascript. So have opted to do what @Bobby Clapp suggested and just make it required. This version, thanks to Deniz Kaya on stackoverflow can also be used for multiple videos.   You can see it in action at https://pembel.co.uk/s/video 
// 2. This code loads the IFrame Player API code asynchronously. // Code thanks to Deniz Kaya at https://stackoverflow.com/questions/20021429/youtube-api-onstatechange-listener-stops-working-after-switch-src-attr// 2. This code loads the IFrame Player API code asynchronously.
// Code thanks to Deniz Kaya at https://stackoverflow.com/questions/20021429/youtube-api-onstatechange-listener-stops-working-after-switch-src-attr
var tag = document.createElement('script');tag.src = "https://www.youtube.com/iframe_api";
var firstScriptTag = document.getElementsByTagName('script')[0];
firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);// 3. This function creates an <iframe> (and YouTube player)
// after the API code downloads.
var player;
function onYouTubeIframeAPIReady() {
player = new YT.Player('player', {
playerVars: { 'autoplay': 1, 'controls': 1},
events: {
'onReady': onPlayerReady,
'onStateChange': onPlayerStateChange
}
});
}var playerReady = false;
// 4. The API will call this function when the video player is ready.
function onPlayerReady(event) {
playerReady = true;
}
// 5. The API calls this function when the player's state changes.
// The function indicates that when playing a video (state=1),
// the player should play for six seconds and then stop.
function floaded1() {player = new YT.Player('player1', {
events: {
'onStateChange': function (event) {
if (event.data == 0) {
jQuery('#field_7d91m').val('fgr32');
};
}
}
});
}