Can you set the video to play only when someone clicks the play button, similar to wistia?

No, the video email experience is similar to the video experiences that work well on Facebook, Instagram, Twitter, etc. - the video starts playing automatically when the email is opened - muted, often with captions. Interested viewers can then click on the video to be taken to a destination of your choice, which can include the video playing with sound, or your product page, whatever you want depending on your marketing objectives.
In this way >98% of your audience enjoys the video content, compared to ~5% if you have to wait for them to click.