First you need to activate your Part B under Medicare. There are 2 forms that need to be completed, one by you and one by your employer that will enable you to have special election period with onto Part B. Both forms, when completed, need to be taken to Social Security.
Once you have Part B, you have a special election to enroll onto a Medigap/Medicare supplement and stand-alone drug plan or onto a Medicare Advantage HMO plan.
Note: Confirm with your employer that the drug benefit you have been on thru the Company is considered creditable drug coverage under Medicare. If it isn’t, then you will carry a penalty for each month you were not on creditable drug coverage and you will be assessed that penalty in perpetuity.