Spec MX-5, LLC has released a chart detailing the three basic ways to participate in the $100k Spec MX-5 Challenge Series. They have also detailed a fourth individual race option for drivers who want to try the series, presumably at a convenient track. Here is the chart:
Applications are still being accepted, but with the East series starting in 6 weeks, time is running out.
For more information on the 2019 Spec MX-5 Challenge Series, check out the FAQ Page.