Where do you add a posting URL so when a user submits a form it will post to our Leads Management System?

You can post the submission data to the thank you page by following this guide: How to Post Submission Data to Thank You Page.

You can also check out our integrations with several crm by following this link: http://apps.jotform.com/category/crm 

Do let us know if you need further assistance.
Regards.