Download Postman from https://www.getpostman.com/.
For creating an image, create a new POST request to https://hcti.io/v1/image.
First, go to the Authorization tab, select "Basic Auth" and enter your User ID and API Key (available from your Dashboard).
Go to "Body" and click "form-data". Then enter your HTML/CSS.
Press send and you'll get back the URL to your generated image.