Once you sign up, you can obtain your API key, by going to the dashboard section
You can call the API with any http client of your choice using the API key. Here are examples of usage for different programming languages / tools :
Command line (using cUrl)
curl -v -o crop_example.jpeg -H "X-Imager-Key: YOUR_API_KEY" --data-binary @jpeg_image.jpg 'https://api.imager200.io/crop/sync?x0=100&y0=150&x1=350&y1=400'
More details can be found in the API docs