as far as I know, this is not possible to get it using HTTP transformation, because there is a Feature Request which is still waiting to be implemented:
CORE-7839 : Need support for Bearer authentication in HTTP transformation.
I'm unsure of any possible workaround.
Probably the only workaround within PowerCenter is to program your own Java Transformation handling the complete process. To me it seems that quite a few customers have done that in the past, there are some questions around related topics every now and then (however, not specifically about the Bearer token, but I'm confident that should be manageable in Java).