I have got a bar with Custom layout Theme in O365.
I need to put Search tab with search box like this:
By design, we can't add search bar to top bar.
As Trevor mentions, you need to add your user to Targeted Release.( Once this feature is released, these users will receive these updates at the first time.)
With this option, you and your users can be the first to see the latest updates and help shape the product by providing early feedback. You can choose to have individuals or the entire organization receive updates early.