I am seeing where it must be on record by Oct 31st, 2021
https://www.freep.com/story/news/local/michigan/2021/12/07/michigan-motorists-auto-insurance-refunds-400-per-vehicle/6414550001/
In a Monday letter to the state insurance agency, MCCA Executive Director Kevin Clinton said all vehicles insured as of Oct. 31, 2021, will receive $400 refunds, with one exception: Vehicles with "historical vehicle" plates will receive refunds of $80, he said.
https://www.clickondetroit.com/news/michigan/2021/12/18/get-caught-up-timeline-faq-released-for-michigan-auto-insurance-refunds/
The refund plan issued by the MCCA will result in $400 refunds sent to policyholders for every vehicle that was insured as of 11:59 p.m. on October 31, 2021.