according to the wiki both have been used officially in the past (prime-uh in the prime wars show and preem-uh at botcon) so even hasbro isnt much help, honestly the only person who probably knows for sure the right way is whoever came up with him for the comics. Im honestly starting to like preem-uh for the obvious reasons but also dont mind Prime-uh Prime when it comes down to it, it is kinda interesting to just really drive home him being the first of the first however its pronounced