How do I push leads in from another system?

Use the campaign's Lead API. Open an outbound campaign and go to its Manage Leads via API tab, which documents Add Lead, Get Leads, Update Lead, and Remove Lead with cURL, JavaScript, and Python samples already pointed at that campaign.

Authenticate with a Bearer token: either the account API token or a scoped credential the account owner mints for that integration. Send it in one header, replacing the example value with your own token:

Authorization: Bearer cmk_0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef

The header name is Authorization. Its value is the word Bearer, then one space, then the token itself. A scoped credential begins cmk_ and is 68 characters long; the account API token is a 64-character string with no prefix. Send the token and nothing else: the server does not strip angle brackets and does not recognise example text, so a placeholder left in place is simply a token that does not exist.

The API includes safeguards: phone numbers are normalized and checked, and requests are rate-limited to 120 per minute per token and 240 per minute per IP. Because add and update match on the phone number, pushing the same lead twice updates it instead of duplicating it.