You can control when (if ever) you intend to enroll in Medicare B (and A) in Settings and Assumptions > Social Security and Medicare. You can choose the age you intend to enroll and you should note as well that the default assumption is that Medicare premiums will go up 3% real (over inflation) each year.
The Medicare B premium is calculated internally based on the IRMAA calculation with reference to your previous two years of earnings. You may need to add a special expense or special receipt to adjust the current year and next year for Medicare Part B premiums. Since MaxiFi doesn’t know your last two years of earnings, it uses your current-year earnings as a proxy for both. For next year and beyond it does know the previous two years earning and the Medicare premiums are correct.