I assume that you are having Office 365 Business Premium or Enterprise E3 subscription as you said that 'I have an office 365 setup with several users using office 365 apps'.
So for using Office 2019 pro plus for one of the user, purchase Office 365 Business Essential or Enterprise E1 for that user (as these subscriptions don't have Office Applications) and install this Office 2019 license on that users' system and connect the system with this users' Office 365 credentials.
You will be getting all the features with security updates included but you won't be getting any new features and updates of major releases and you have to pay the full amount when a new major release comes but with Office 365 apps you will always have the latest version as it is subscription based.
Best Response confirmed by
robertdh (Occasional Contributor)
I agree with what's already been said, you'd only use Office 2019 with a plan that doesn't include the Office 365 desktop apps already.
This is because the version of the apps that come with Office 365 (for those plans that include the desktop apps) has more features, Microsoft went as far as saying recently “Office 365 crushes Office 2019”. I have put more details here if it is of interest - Office 365 ProPlus and Office 2019 Comparison.
There isn't a licence to assign as such for Office 2019, you'd just activate Office 2019 on the PC where it's going to be used. This can vary depending on how Office 2019 was bought but this is the main article that goes through the option - Activate Office.