Hitting the application manually

For authenticated endpoints, HTTP Token Authentication is used - by default, locally it's set to my-secret-key.

curl \
  -X POST \
  -H "Authorization: Token my-secret-key" \
  -H "Content-type: application/json" \
  -d '{
    "template":{
      "subject": "foo",
      "content": "bar",
      "template_type": "letter"
    },
    "values": null,
    "letter_contact_block": "baz",
    "filename": "hm-government"
  }' \
  http://localhost:6013/preview.pdf

template is an object containing the subject and content of the letter, including any placeholders
values is an object containing the keys and values which should be used to populate the placeholders and the lines of the address
letter_contact_block is the text that appears in the top right of the first page, can include placeholders
filename is an absolute URL of the logo that goes in the top left of the first page (must be an SVG image)