Initialize test run with Xray API

How do I initialize Xray test execution run through the api

now what I get when I call:
https://jira.betware.com/rest/raven/1.0/api/testrun/?testExecIssueKey=BWITREXTRU-327&testIssueKey=BWITREXTRU-195

{ id: 11703, status: "TODO", defects: [ ], evidences: [ ], steps: [ ] }

steps are unfortunately empty, so I can not fill in there corresponding rsuslts