product adding a mailing list sign up option to a custom form at checkout

BACKGROUND

My clients website is a shop for selling international art tours. Each tour is set up:

an event

A service product for tour deposit and;

A service product for final payment (password protected as the client doesn't want their itineraries to be made public)

I'm trying to get tour attendees to sign up to their specific tour mailing list (from the deposit purchase stage) so my client can send email campaigns to them specifically for tour correspondence until departure.

ISSUE

I can't find a way to ensure that at deposit product checkout, they tick their agreement and offer their email for sign up to their tours mailing list.

I've considered trying checkout forms - but they only allow for sign up to ONE mailing list (as far as im aware anyway)

I've not found any form field in the product custom form area to add a mailing list subscription there either.

PLEA FOR URGENT HELP!

can anyone help?!