Lead Assign has an API so you can POST leads into an account. (Note that the usual way to get leads in is via the email endpoint – simply email leads into your company endpoint as listed in the Company Settings section).
For more on our API click Here
We also have a Facebook lead Ad program to directly input leads from FB Lead Ads directly into Lead Assign. To view integration options and information specific to your account please visit the “Integrations” page in app.