From a previous post on here, I found a setting in the 365 admin area.
Settings > Org Settings > Organization Profile > Release Preferences > Select release type.

You can use this for everyone or selected users. New Outlook wasn't immediately updated, so I checked it the following day and I now have the feature.
I must say though... this was a feature that was holding me back from using new outlook, but having used for a couple of days with this feature, it is absolutely awful. Stick with classic outlook. I really hoped it would be a copy of OWA, but it's not. It all looks and feels dreadful, still missing some features (like quick actions for group mailboxes) and duplication of information (i.e. attachments appear twice in emails - why?! 😅)
If you like it though and need this feature - the settings above should get it working.