Create the API site in Salesforce.
Create a new Connection of type Destination in Sitecore CDP. Use None for authentication, and enter the public API endpoint you enabled above with POST method. Additionally add an x-api-key HTTP header with the value that matches the one in the CDP4S settings.
The Salesforce endpoint is heavily restricted for security purposes. It:
requires a matching API key header,
allows only a simple JSON browser ref parameter as input,
performs callbacks to Sitecore CDP to verify the browser ref and retrieve Guest data,
never returns any output.