Since 2017, Browsers have required users "interact" with a website in some way before videos will be allowed to auto-play. You can modify Chrome to allow for auto-play, but it's not super easy to do.
In OBS and the Electron Capture app, auto-play is allowed.
VMix v77 does not support auto-play, which must be an oversight on their behalf... right?? haha. A complaint has been filed regardless; voice your own request for the feature here: https://forums.vmix.com/posts/t22181-CEF-V77-browser-setting.
A full-screen-sized play button will show up to help accommodate the need for user gestures before auto playing videos.