By inserting Merge Tags into your Email Templates.
You can use the following that will ensure that the link is included in the customer's email:
{{ TRANSACTION.PAYMENT_LINK }}
If the Reservation has an expiry date and time, then you can include the following if statement so that the customer knows when they have to pay by:
{% if TRANSACTION.EXPIRES_AT %}
{{ TRANSACTION.EXPIRES_AT.strftime('%a %d %b %Y %H:%M') }}
{% endif %}