not getting extended healthcare benefits

Hi, I know that my company does offer a group benefits plan with extended healthcare to full time employees (I've seen it in job postings for another department). However, I was previously working as a part-timer (so I wasn't expecting anything before) before they offered for me to become a fulltimer working 40 hours a week. So I accepted and they sent me a letter which said I would be guaranteed the 40 hours a week as well as vacation time.

But I didn't see them mention anything about extended healthcare benefit which I want. I'm not sure if they require me to work a bit longer as a full-timer first or something, there wasn't much information. How do I go about addressing this? Should I ask HR or my supervising manager?

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