To make a 'create instance' API call you need some specific information such as the workflow ID and any merge field data. You can locate this in your HelloWorks portal by navigating to the 'API Request Info' tab in the main menu on the left side of the page. From here simply choose the workflow name and a dropdown will provide you the information you need. For more information check out the API Documentation.
There is also an endpoint for the above information as well.