This can be done by adding a question to the registration form, setting up a question group, and using the answer to the question in the email’s To: field. First you add a new question for “Booking Contact Email Address”, set the question type to Email, and set the question to be required.
(In Event Espresso → Registration Forms)
Then, you add a “Booking Contact Info” question group. You’ll assign the Booking Contact Email Address” question to the question group.
Then you go to edit the event and you’ll set the group to appear for the primary attendee only.
Finally, you can set up the Primary Registrant message’s by adding a shortcode to the email’s TO: field: [RECIPIENT_ANSWER_*Booking Contact Email Address]
(In Event Espresso → Messages → Default Message Templates)