The format is something like this http://prntscr.com/ioe53y, but if you want have an optimal result you should combine pex with a good essentials plugin like ultimatecore or nucleus.
For the prefix in the tab and on the player head you can use ultimatecore or a specific plugin that do that, like TabModifier the problem is that it requires luckperms.
If you decide to switch to luckperms don’ worry it has an import feature that allows you to import your permissionex perms.
Hope this will help you 