(I'm assuming that you're using FlowPlayer 5.)
To bind JavaScript functions to FlowPlayer events, you need to get a hold of the API for your given player, as is described here: https://flowplayer.org/docs/api.html
Among other things, the page contains a list of all available events. Notably, there is no "start" event, but I've found that "load" does the job just fine. "finish" and "unload" seem to do the same in the setups I've tried so far (I'm using the splash screen configuration exclusively).
An example of that might look like this:
<script type="text/javascript">
// bind your player to a var
var player = $('#something');
// initialize the player
player.flowplayer();
// bind the api to a var - you can do this only after you have initialized it
var api = player.data('flowplayer');
// now we can bind events to the player via the api
api.bind('load', function(){
// do something when the video starts
})
</script>