There's a couple different ways you could do this depending on how your landing page is setup.
If you are using Marketo's video element (Add a Video - Marketo Docs - Product Documentation ) then you should be able to add ?autoplay=1 to the URL as shown here: Embedded Video with Autoplay Not Pulling Through on Landing Page .
If you are using a Rich Text element or HTML element in the landing page, then you'll need to add the autoplay attribute to the embed code: Embed videos & playlists - YouTube Help
If you are using a landing page variable to insert the video URL into a <video> element on the page you'll need to use the attributes of the <video> element to control autoplay as discussed here: Auto play video in Marketo landing pages
HOWEVER...be aware that all of those methods above are subject to the visitor's browser settings. For example, last year Google Chrome began preventing video autoplay: Google Chrome version 66 will stop videos from autoplaying
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