When you create your new offering (My data offerings > Add an offering), select "Ceate an offering via API" among the three available options. 

To create an offering via API, you must provide the same information as you would for a data offering by file:

• Description of your offering and your data
• Your offering's visibility
• Whether data with personal information is present
• License contract terms
• Data pricing

For an offering via API, you must also provide:

• The description of your API (name, URL, output format, parameters)
  How should I describe my API?
• Conditions for accessing the API (authentication method, send method)
  How do I validate the conditions for accessing my API?

When all the information has been provided, test access to each URL before continuing to load and publish your offering.
How can I verify access to the URLs in my API?