All you'll need to do to book a Brightline train connection on JetBlue is go to the airline's website or mobile app and plug in the city (not necessarily the airport) you're hoping to visit.
For example, let's say I wanted to travel from New York's John F. Kennedy International Airport (JFK) to Aventura.
Though the JetBlue's website you would select your itinerary: which includes flights from JFK to Orlando, and then a connection to a Brightline train segment (you can see where it says "operated by Brightline"). as you can see below.
JetBlue-Brightline connections will include complimentary shuttle service between airports and nearby stations.