curl --request GET \
  --url http://{host}:{port}/{contextPath}/process-definition/key/{key}/xml \
  --header 'Authorization: Basic <encoded-value>'

{
  "id": "anProcessDefinitionId",
  "bpmn20Xml": "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<definitions\n                         xmlns=\"http://www.omg.org/spec/BPMN/20100524/MODEL\"\n                         xmlns:camunda=\"http://camunda.org/schema/1.0/bpmn\"\n                         targetNamespace=\"Examples\">\n                         <process id=\"oneTaskProcess\" isExecutable=\"true\">\n                           <startEvent id=\"theStart\" />\n                           <sequenceFlow id=\"flow1\" sourceRef=\"theStart\" targetRef=\"theEnd\" />\n                           <endEvent id=\"theEnd\" />\n                         </process>\n                       </definitions>"
}