Using API to automate new member emails through a third party service

[Cyn]

We would like to integrate our new registration emails into our existing newsletter email service. We want to have all new registration emails electing to opt in to our newsletters sent to our email service provider to be placed in a list which automatically sends out a welcome email to the new member. They tell us this can be achieved through API and what they need from vBulletin is the SLID number. Vbulletin sent me here since it involves modifying the code. Anyone have any info for me to make this happen?

Thanks!
Cynthia