With HeySummit, you can run an event with a mixture of live and pre-recorded talks. In this case, you have chosen to use pre-recorded talks. Pre-recorded sessions start automatically at their scheduled start time.
Head to Content > Talks. Click on the '...' next to the talk you want to set up, select Manage Talk and choose Pre-Recorded.
From there, choose Pre-Recorded and click next. Click on the provider you want to use and add the video ID. You can also add the Video URL or Embed Code.
Fill in all the information about your talk then click Save.
For tips on using the HTML5 Video option within this setting, take a look at this article.