Hey
I doubt you'll come across an
Entheobotany course in any accredited university anywhere within the next ~15 years...
Ethnobotany yes (the interaction of humans and plants in culture - it's an overarching field, studying things from how the spread of modern wheat changed cultures to, yes, entheogenic plants and their cultural and religious uses).
Anyway, this level of specialization is more a grad program or even PhD thing, so you'll have to pick an entry point into science. In my opinion your best bets are:
Cultural anthropology - It's a field of hermeneutic study of human cultures and subcultures via close observation. Most of the pioneers who first discovered the use of entheogenic plants were cultural anthropologists studying native cultures. It's a good choice if you see yourself talking to native shamans in the jungle ten years from now.
Botany or Agriculture - Well it's just that. Plant biology, or the science of growing plants for human use. If you want to focus more on the plants than the culture around them, one of these might give you the expertise needed.
Psychology or Psychiatry - Especially if you want to focus on entheogens in Western society and the War on Drugs. You can specialize in addictology, maybe officially study the psychological or neurological effects of hallucinogens, and generally have the chance to become an authority figure able to sway legislators and public opinion.
Organic chemistry - If you prefer to have a close relationship with the chemicals themselves, understand extraction and synthesis, and maybe follow in Shulgin's footsteps, this is also a possible route.
But no matter which you choose for your undergrad, you'll eventually have the ability to branch out and focus in on your interests at grad level.
