Hi Andrew,
Thanks for your question!
You can't modify the confirmation email sent to the customer but you can edit the "reminder email".
If you go into "Services" in the left nav and click on your services, there is an option below "Custom Fields" to "Add an email reminder" - You can tailor the message in that email with the URL you need and make it so Bookings sends it up to 2 weeks in advance. So you could make it so that your customers can only book up to 2 weeks in advance and set this reminder to go up to 2 weeks in advance, your customers will get the reminder email almost immediately after they make the booking.
Hope this helps!
Gabriel.