All you have to do is create a video that promotes your lead magnet, then create a Facebook post with your video and a clear CTA (Call to Action) that has a link to your lead magnet landing page.
Then go to the Audiences section of Facebook business manager and create a new audience that is composed of anyone who watches your video for more than x seconds (thru play). Save this audience to use to send traffic to later when you are ready to advertise.
The best part is that it is quick and easy once you have your lead magnet and landing page setup.
FREE Facebook Video Views + Custom Audience Worksheet
Example shown below